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  • Articles  (4,445)
  • Animals  (2,869)
  • Theoretical, Physical and Computational Chemistry  (1,576)
  • AERODYNAMICS
  • 1995-1999  (4,445)
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Year
Journal
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  • 1
    Electronic Resource
    Electronic Resource
    Segmentation and tracking of piglets in images (1995)
    Springer
    Machine vision and applications 8 (1995), S. 187-193 
    Details
    ISSN: 1432-1769
    Keywords: Tracking ; Segmentation ; Pigs ; Animals ; Computer vision
    Source: Springer Online Journal Archives 1860-2000
    Topics: Computer Science
    Notes: Abstract An algorithm was developed for the segmentation and tracking of piglets and tested on a 200-image sequence of 10 piglets moving on a straw background. The image-capture rate was 1 image/140 ms. The segmentation method was a combination of image differencing with respect to a median background and a Laplacian operator. The features tracked were blob edges in the segmented image. During tracking, the piglets were modelled as ellipses initialised on the blobs. Each piglet was tracked by searching for blob edges in an elliptical window about the piglet's position, which was predicted from its previous two positions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01215814
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  • 2
    Electronic Resource
    Electronic Resource
    Phylogenetic position of Dictyostelium inferred from multiple protein data sets (1995)
    Springer
    Journal of molecular evolution 41 (1995), S. 238-246 
    Details
    ISSN: 1432-1432
    Keywords: Cellular slime molds ; Animals ; Fungi ; Plantae ; Maximum-likelihood method ; Evolution
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract The phylogenetic position of Dictyostelium inferred from 18S rRNA data contradicts that from protein data. Protein trees always show the close affinity of Dictyostelium with animals, fungi, and plants, whereas in 18S rRNA trees the branching of Dictyostelium is placed at a position before the massive radiation of protist groups including the divergence of the three kingdoms. To settle this controversial issue and to determine the correct position of Dictyostelium, we inferred the phylogenetic relationship among Dictyostelium and the three kingdoms Animalia, Fungi, and Plantae by a maximum-likelihood method using 19 different protein data sets. It was shown at the significance level of 1 SE that the branching of Dictyostelium antedates the divergence of Animalia and Fungi, and Plantae is an outgroup of the Animalia-Fungi-Dictyostelium clade.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00170678
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  • 3
    Electronic Resource
    Electronic Resource
    Evolutionary relationships of eukaryotic kingdoms (1996)
    Springer
    Journal of molecular evolution 42 (1996), S. 183-193 
    Details
    ISSN: 1432-1432
    Keywords: Small-subunit ribosomal RNA ; Phylogeny ; Animals ; Fungi ; Plants ; Alveolates ; Heterokonts ; Stramenopiles
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract The evolutionary relationships of four eukaryotic kingdoms—Animalia, Plantae, Fungi, and Protista—remain unclear. In particular, statistical support for the closeness of animals to fungi rather than to plants is lacking, and a preferred branching order of these and other eukaryotic lineages is still controversial even though molecular sequences from diverse eukaryotic taxa have been analyzed. We report a statistical analysis of 214 sequences of nuclear small-subunit ribosomal RNA (srRNA) gene undertaken to clarify these evolutionary relationships. We have considered the variability of substitution rates and the nonindependence of nucleotide substitution across sites in the srRNA gene in testing alternative hypotheses regarding the branching patterns of eukaryote phylogeny. We find that the rates of evolution among sites in the srRNA sequences vary substantially and are approximately gamma distributed with size and shape parameter equal to 0.76. Our results suggest that (1) the animals and true fungi are indeed closer to each other than to any other “crown” group in the eukaryote tree, (2) red algae are the closest relatives of animals, true fungi, and green plants, and (3) the heterokonts and alveolates probably evolved prior to the divergence of red algae and animal-fungus-green-plant lineages. Furthermore, our analyses indicate that the branching order of the eukaryotic lineages that diverged prior to the evolution of alveolates may be generally difficult to resolve with the srRNA sequence data.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02198844
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  • 4
    Electronic Resource
    Electronic Resource
    Animal Consciousness and Ethics in Asia and the Pacific (1997)
    Springer
    Journal of agricultural and environmental ethics 10 (1997), S. 249-267 
    Details
    ISSN: 1573-322X
    Keywords: Animals ; Asia ; consciousness ; Australia ; Hong Kong ; India ; Israel ; Japan ; New Zealand ; The Philippines ; Russia ; Singapore ; Thailand
    Source: Springer Online Journal Archives 1860-2000
    Topics: Energy, Environment Protection, Nuclear Power Engineering , Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition , Philosophy
    Notes: Abstract The interactions between humans, animals and the environment have shaped human values and ethics, not only the genes that we are made of. The animal rights movement challenges human beings to reconsider interactions between humans and other animals, and maybe connected to the environmental movement that begs us to recognize the fact that there are symbiotic relationships between humans and all other organisms. The first part of this paper looks at types of bioethics, the implications of autonomy and the value of being alive. Then the level of consciousness of these relationships are explored in survey results from Asia and the Pacific, especially in the 1993 International Bioethics Survey conducted in Australia, Hong Kong, India, Israel, Japan, New Zealand, The Philippines, Russia, Singapore and Thailand. Very few mentioned animal consciousness in the survey, but there were more biocentric comments in Australia and Japan; and more comments with the idea of harmony including humans in Thailand. Comparisons between questions and surveys will also be made, in an attempt to describe what people imagine animal consciousness to be, and whether this relates to human ethics of the relationships.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1007774409131
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  • 5
    Electronic Resource
    Electronic Resource
    Unexpected rate enhancement in the intramolecular carboxylic acid-catalyzed cleavage of o-carboxybenzohydroxamic acid (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 216-222 
    Details
    ISSN: 0894-3230
    Keywords: o-carboxybenzohydroxamic acid ; phthalic anhydride ; phthalic acid ; hydrolysis ; intramolecular acid catalysis ; kinetics ; activation parameters ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Phthalic anhydride was detected spectrophotometrically in the hydrolysis of o-carboxybenzohydroxamic acid (OCBA) in CH3CN-H2O solvent containing 0.03 mol dm-3 HCl. Pseudo-first-order rate constants (k1) for hydrolysis of OCBA are almost independent of the change in CH3CN content from 10 to 80% (v/v) in mixed aqueous solvents. The rate constants k1 are more than 10-fold larger than the corresponding rate constants for hydrolysis of phthalamic acid. These observations are explained in terms of a mechanism slightly different from the mechanism for hydrolysis of phthalamic acid. The activation parameters, ΔH* and ΔS*, are not affected appreciably by an increase in CH3CN content from 10 to 80% in mixed aqueous solvents. © 1998 John Wiley & Sons, Ltd.
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  • 6
    Electronic Resource
    Electronic Resource
    Solute-solvent and solvent-solvent interactions in binary solvent mixtures. Part 7. Comparison of the enhancement of the water structure in alcohol-water mixtures measured by solvatochromic indicators (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 185-192 
    Details
    ISSN: 0894-3230
    Keywords: solute-solvent interactions ; solvent-solvent interactions ; binary solvents ; alcohol-water mixtures ; water structure ; solvatochromic indicators ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: A preferential solvation model that takes into account the enhancement of the structure of water when small amounts of alcohol are added was applied to solvatochromic data for binary mixtures of water with 2-methylpropan-2-ol, propan-2-ol, ethanol and methanol. Application of the model allows the calculation of the effect of the enhancement of the water structure on solvatochromic solvent properties. It is demonstrated that the enhancement of water structure increases the solvent dipolarity/polarizability and hydrogen-bond donor acidity and decreases the solvent hydrogen-bond acceptor basicity. The effect decreases in the order 2-methylpropan-2-ol-water, propan-2-ol-water, ethanol-water and methanol-water. © 1998 John Wiley & Sons, Ltd.
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  • 7
    Electronic Resource
    Electronic Resource
    Structural dependence of heterolytic bond dissociation energy of carbon-carbon σ bonds in hydrocarbons (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 157-170 
    Details
    ISSN: 0894-3230
    Keywords: heterolytic bond dissociation energy ; carbon-carbon σ bonds ; hydrocarbons ; structural dependence ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The coordination of a resonance-stabilized hydrocarbon cation and anion yielded a series of unprecedented hydrocarbons, which are susceptible to thermal heterolytic cleavage of carbon-carbon σ bonds in polar media, generating the original ions under reversible conditions. When the component ions were sufficiently stabilized, some ion pairs were even isolated as solids, thus providing the first examples of hydrocarbon salts. The direct observation of the heterolysis by means of spectroscopy permitted reliable thermodynamic treatments of the observed degree of ionic dissociation. Correlation of the free energy of heterolysis with solvent dielectric constants and parameters of ion stabilities such as pKHA, pKR+ and redox potentials revealed the importance of the thermodynamic stabilities of ions, the degree of solvation and steric congestion in the starting molecule as major controlling factors in the heterolysis. © 1998 John Wiley & Sons, Ltd.
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  • 8
    Electronic Resource
    Electronic Resource
    Correlation of the rates of solvolysis of the N,N-diphenylcarbamoylpyridinium ion (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 273-276 
    Details
    ISSN: 0894-3230
    Keywords: N,N-diphenylcarbamoylpyridinium ion ; solvolysis ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Solvolyses of the N,N-diphenylcarbamoylpyridinium ion are subject to specific and/or general base catalysis, which can be eliminated by addition of perchloric acid or increased, especially in fluoroalcohol-containing solvents, by addition of pyridine. The uncatalyzed solvolyses in aqueous methanol and aqueous ethanol involve a weakly nucleophilically assisted (l  =  0.22) heterolysis and the solvolyses in the pure alcohols are anomalously slow. © 1998 John Wiley & Sons, Ltd.
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  • 9
    Electronic Resource
    Electronic Resource
    Effects of ionic and non-ionic micelles on rate of hydroxide ion-catalyzed hydrolysis of securinine (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 209-215 
    Details
    ISSN: 0894-3230
    Keywords: securinine ; cationic micelles ; anionic micelle ; non-ionic micelle ; alkaline hydrolysis ; kinetics ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The effects of micelles of cetyltrimethylammonium bromide (CTABr), tetradecyltrimethylammonium bromide (TTABr), sodium dodecyl sulfate (SDS) and polyoxyethylene 10 lauryl ether (C12E10) on the rates of alkaline hydrolysis of securinine were studied at a constant [-OH] (0.05 M). An increase in the total concentrations of CTABr, TTABr, SDS and C12E10 from 0.0 to 0.2 M causes a decrease in the observed pseudo-first-order rate constants (kobs) by factors of ca 2.5, 3, 7 and 4, respectively. The observed data are explained in terms of pseudophase and pseudophase ion-exchange (PIE) models of micelle. The binding constants, KS, of securinine with SDS, C12E10, CTABr and TTABr micelles are 32.4, 14.8, 22.1, and 9.1 M-1, respectively. The magnitudes of the second-order rate constants, kM, for the reactions in the micellar pseudophase are negligible compared with the corresponding rate constant, kW, for the reaction occurring in the aqueous pseudophase for CTABr, TTABr, SDS and C12E10. © 1998 John Wiley & Sons, Ltd.
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  • 10
    Electronic Resource
    Electronic Resource
    Colloidal CdS-induced photocatalytic reaction of 2-methylindole - mechanistic analysis of oxidation of indoles (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 277-282 
    Details
    ISSN: 0894-3230
    Keywords: colloidal CdS-induced photochemical reactions ; photocatalytic reactions ; photooxidation ; 2-methylindole ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: 2-Methylindole (2-MI) is adsorbed on the surface of colloidal CdS particles with an adsorption intensity of 0.6 × 103 dm3 mol-1. A new emission band at 530 nm is produced by forming an exciplex between excited CdS and 2-MI and the red emission due to CdS is simultaneously quenched. The emission maxima of green bands for different indoles increase in the order indole 〈 tryptophan 〈 2-MI 〈 3-MI and are observed at 508, 520, 530 and 540 nm, respectively. The shift in emission maxima is related to the oxidation potential of these substrates. The irradiation of an aerated reaction mixture containing CdS and 2-MI with visible light induces the oxidation of adsorbed 2-MI by photogenerated holes to produce 2-methyl-3-indolinone and 2-acetamidobenzaldehyde. The latter product is formed due to oxidative C - C bond cleavage of the pyrrole ring. The reactivity of trapped holes towards the adsorbed 2-MI is evidenced by a decrease in the lifetime of the red emission of CdS in the presence of 2-MI. In this reaction the possibility of the participation of singlet oxygen is ruled out. A general mechanism of CdS-induced oxidation of indoles is discussed. © 1998 John Wiley & Sons, Ltd.
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  • 11
    Electronic Resource
    Electronic Resource
    The Menschutkin reaction of benzyl p-toluenesulfonates with N,N-dimethylanilines. Evidence for the duality of SN1 and SN2 mechanisms (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 254-260 
    Details
    ISSN: 0894-3230
    Keywords: Menschutkin reaction ; duality of SN1, SN2 mechanisms ; benzyl tosylates ; Yukawa-Tsuno equation ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The rate data for the Menschutkin reaction between strongly activated Z-substituted benzyl p-toluenesulfonates and Y-substituted N,N-dimethylanilines in acetonitrile at 35 °C fit the equation, kobs  =  k1 + k2 [DMA], which is consistent with concurrent first- and second-order processes. The k1 and k2 values for each substrate were separated based on the above equation. The SN1 rate constant, k1, is unaffected by the nature of the nucleophile, whereas the SN2 rate constant, k2, increased with the electron-donating substituent of the N,N-dimethylaniline. The substituent effect on the k1 values is linearly correlated by the Yukawa-Tsuno equation with ρ  =  -5.2 and r  =  1.3. The unimolecular reaction can be regarded as a classical SN1 mechanism. In contrast, that on the k2 values shows an upward curvature when analyzed by the Brown σ+ treatment. These results are ascribed to the simultaneous and independent occurrence of SN1 and SN2 mechanisms in the present Menschutkin reaction. © 1998 John Wiley & Sons, Ltd.
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  • 12
    Electronic Resource
    Electronic Resource
    Editorial (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 231-231 
    Details
    ISSN: 0894-3230
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: No Abstract
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  • 13
    Electronic Resource
    Electronic Resource
    Solvent effects on infrared spectroscopic and calorimetric characteristics of aliphatic ketones in binary solvent mixtures (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 283-289 
    Details
    ISSN: 0894-3230
    Keywords: calorimetry ; IR spectra ; ketones ; solvation ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Solution enthalpies of n-hexane, acetone, butan-2-one and octan-2-one in a series of tetrachloromethane-acetonitrile mixtures and the carbonyl stretching absorption frequencies in the IR spectra of these ketones were determined. It was found that over the whole range of concentrations (varying from neat tetrachloromethane up to neat acetonitrile) the solvation enthalpy of these compounds can be obtained additively from the contributions of the alkyl and carbonyl fragments. The solvent effect on the solvation enthalpy of the carbonyl group was found to be satisfactorily correlated with the corresponding IR frequency shifts of the C=O group. It was also found that the sensitivities of the carbonyl IR frequencies to the solvent composition are different for various ketones. From both IR and calorimetric data, the preferential solvation parameters were evaluated. The differences between the IR spectroscopic and calorimetric data are discussed. © 1998 John Wiley & Sons, Ltd.
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  • 14
    Electronic Resource
    Electronic Resource
    Solvent effects on C=O stretching frequencies of some 1-substituted 2-pyrrolidinones (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 261-272 
    Details
    ISSN: 0894-3230
    Keywords: 1-substituted 2-pyrrolidinones ; C=O stretching frequencies ; solvent effects ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: In an effort to model solute-solvent interactions, the C=O stretching frequencies of five 1-substituted 2-pyrrolidinones and four other carbonyl-containing compounds were measured for 30 common solvents. These were then correlated with four empirical parameter sets and one theoretical (computational) parameter set. While an empirical parameter set gave the best correlation equations, the theoretical parameter equations are physically and statistically significant. Solvent volume, polarizability and hydrogen bond donor acidity (capacity) terms are significant in the correlation equations. © 1998 John Wiley & Sons, Ltd.
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  • 15
    Electronic Resource
    Electronic Resource
    Molecular orbital calculations on the P - S bond cleavage step in the hydroperoxidolysis of nerve agent VX (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 232-240 
    Details
    ISSN: 0894-3230
    Keywords: P - S bond cleavage ; hydroperoxidolysis ; nerve agent VX ; molecular orbital calculations ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The P - S bond cleavage process in the hydroperoxidolysis of a model system for the nerve agent VX was studied using ab initio and semiempirical molecular orbital methods. Aqueous solvation effects were included through single-point calculations using the semiempirical SM5.2PD/A continuum solvation model and geometries optimized at the HF/MIDI! level of theory. The predominant pathway for P - S bond cleavage involves pseudorotation of a low-energy trigonal bipyramidal intermediate followed by apical ligand ejection. In aqueous solution, the free energy barriers for these processes are found to be 14.3 and 4.6 kcal mol-1, respectively, with electronic energies calculated at the MP2/cc-pVDZ//HF/MIDI! level of theory. By comparison with another continuum model of solvation (PCM), it is concluded that the SM5.2PD/A model performs well even for hypervalent phosphorus species, in spite of not having included any such molecules in the model's parameterization set. © 1998 John Wiley & Sons, Ltd.
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  • 16
    Electronic Resource
    Electronic Resource
    Conformational analysis of a 12-membered crown dithioether in the solid state and in solution (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 241-253 
    Details
    ISSN: 0894-3230
    Keywords: crown dithioether ; conformational analysis ; multicomponent equilibrium ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The solid-state molecular structure and the conformational behaviour in solution of the 12-membered crown dithioether 8-methyl-1,4-dioxa-7,10-dithiacyclododecane-5,12-dione were studied by x-ray crystallography, 1H and 13C NMR spectroscopy and molecular mechanics. The conformational rigidity of some constituent structural fragments allowed a detailed analysis of the structure and distribution of the conformers. A protocol for studies of multiconformational equilibrium was developed by means of the combined use of structure calculations and dynamic NMR measurements. © 1998 John Wiley & Sons, Ltd.
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  • 17
    Electronic Resource
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    Metal-catalyzed carbenoid reactions with iodonium and sulfonium ylides (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 321-333 
    Details
    ISSN: 0894-3230
    Keywords: metal-catalyzed carbenoid reactions ; iodonium ylides ; sulfonium ylides ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Transition metal-catalyzed decomposition of phenyliodonium and diphenylsulfonium ylides was investigated with regard to application in asymmetric carbenoid reactions. Phenyliodonium ylides react in the presence of Rh(II) catalysts with the same selectivity in inter- and intramolecular cyclopropanations as the corresponding diazo compounds, and intramolecular CH insertions proceed with identical enantioselectivities. With diphenylsulfonium ethoxycarbonylmethylide the Cu(I)-catalyzed cyclopropanation of olefins affords trans/cis ratios and asymmetric inductions identical with those of diazo compounds, but with Rh(II) catalysts some small, although significant, selectivity variations occur, which are ascribed to coordination of diphenyl sulfide to one of the coordination sites of the catalyst. © 1998 John Wiley & Sons, Ltd.
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  • 18
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    In vivo formation of C - S bonds in biotin. An example of radical chemistry under reducing conditions (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 529-535 
    Details
    ISSN: 0894-3230
    Keywords: biotin ; in vivo C - S bond formation ; radical chemistry ; reducing conditions ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The last step in the biosynthesis of biotin involves the formation of carbon-sulfur bonds at non-activated carbons catalyzed by biotin synthase. S-Adenosylmethionine (AdoMet) and an electron source are essential for activity in cell-free systems. This important finding connects biotin synthase to a family of enzymes, namely pyruvate-formate lyase, anaerobic ribonucleotide reductase and lysine 2,3-aminomutase, which use the same cofactors. Additional experimental data led to the proposition of the following general mechanism. The carbons to be functionalized are first activated by homolytic cleavage of the C - H bonds, initiated by the deoxyadenosyl radical produced by a monoelectronic reductive cleavage of AdoMet with NADPH as electron source. The electron transfer system involves flavoproteins and very likely the [Fe - S] center of biotin synthase. NADPH and the flavoproteins can be replaced by photoreduced deazaflavin. By using a deuterated substrate, a deuterium transfer into deoxyadenosine has been observed, indicating that biotin synthase should be closely related to lysine 2,3-aminomutase, which uses AdoMet as a surrogate of vitamin B12. The source of sulfur, the nature of the immediate sulfur donor and hence the mechanism of trapping of the intermediate radicals are still unknown. © 1998 John Wiley & Sons, Ltd.
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  • 19
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    Ground- and excited-state properties of some 3,4-dihydro-1-(2-p-substituted benzylidene)naphthalenones: substituent and environmental effects (1997)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 687-696 
    Details
    ISSN: 0894-3230
    Keywords: bsorption ; fluorescence ; solvatochromism ; hydrogen bonding ; dipole moment ; naphthalenones ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: ---Electronic absorption and steady-state fluorescence emission of seven 3,4-dihydro-1-(2-p-substituted benzylidene)naphthalenones (1-7) show sizable solvent dependence. The charge-transfer (CT) absorption maxima of these compounds in various solvents show a red shift for the electron-donating substituted compounds (1-5), whereas a blue shift is observed for compounds possessing electron-withdrawing substituents (6, 7). Excitation into the lowest energy absorption gives emission from the locally excited state, which relaxes to the emitting intramolecular charge-transfer state for compounds with strong electron-donating substituents (1-3) in hydroxylic solvents. However, in moderately polar solvents, dual emissions are observed for these compounds. No CT emission is observed for the compounds with moderate electron-donating substituents (3 and 4) or those with electron-withdrawing substituents (6 and 7). Compound 1 with an -N(CH3)2 substituent shows an excited-state dipole moment of 17 D. © 1997 John Wiley & Sons, Ltd.
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  • 20
    Electronic Resource
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    Chemistry of secologanin. Part 5. Graphical analysis of the acidic deglycosylation of vincoside derivatives (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 622-631 
    Details
    ISSN: 0894-3230
    Keywords: secologanin ; vincoside derivatives ; acidic deglycosylation ; graphical analysis ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Acidic hydrolysis and cyclization were studied in vincoside glycosides (‘natural’ series) and their dihydro derivatives (‘dihydro’ series) in which either one or both N atoms were free or blocked by an alkyl group. For interpretation of the results, a graph was constructed in which 25 points (actually circles) represent a maximum of 81 aglycone types and 40 arrows indicate 131 possible cyclizations. The reaction matrix of the graph was under thermodynamic control and in most cases afforded the thermodynamically most stable product aglycones. In addition to the deglycosylation, two types of cyclization were observed. In azacyclizations, the preferred nucleophilic site is N-4 over N-1, and the preferred electrophilic site is C-22 in the glycosides, C-21 over C-19 and C-17 in aglycones. In oxacyclizations, the preferred nucleophilic site is O-17 over C-18 and C-21, and the preferred electrophilic site is C-19 over C-21 and C-17 in the ‘natural’ series, C-21 over C-17 in the ‘dihydro’ series. In one case, the kinetically favoured aglycone types which had been generated in the reaction mixture were trapped in a subsequent reaction (outside the graph) before thermodynamic equilibrium was attained. With the help of graphical analysis it was possible to justify the formation of the most favourable and actually isolated products and pathways out of a large number of possibilities. © 1998 John Wiley & Sons, Ltd.
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    Reactant-solute encounters in aqueous solutions studied by kinetic methods: hydration cosphere overlap and camouflage effects (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 841-846 
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    ISSN: 0894-3230
    Keywords: reactant-solute encounters ; aqueous solutions ; water ; kinetic methods ; hydration cosphere destructive overlap ; camouflage effects ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Rates of chemical reactions in aqueous solutions are often sensitive to low concentrations of added solutes such as ureas, alcohols, α-amino acids and carbohydrates. In this work, several simple chemical reactions were used to probe this sensitivity, which arises from interactions between added solute and the reacting solute. It was found that derived interaction parameters provide a novel insight into interactions between solute molecules in aqueous solution. For example, it was possible to identify two interesting effects, which are termed (i) ‘destructive overlap’ of hydration cospheres and (ii) ‘camouflage effects’ by water-solute interactions. Copyright © 1998 John Wiley & Sons, Ltd.
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    Absolute proton affinities of some substituted toluenes: the additivity rule of thumb for ipso attack (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 663-669 
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    ISSN: 0894-3230
    Keywords: ipso proton affinities ; additivity ; disubstituted toluenes ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The problem of the ipso protonation of toluene and its predominantly disubstituted derivatives was considered by the MP2(fc)/6-31G**//HF/6-31G*+ZPE(HF/6-31G*) theoretical model. The substituents involved covered a wide range of different donor-acceptor capabilities. It is shown that the calculated MP2 ipso proton affinities of substituted toluenes follow mutatis mutandis the same additivity rule which was found earlier to be operative in polysubstituted benzenes, naphthalenes and biphenylenes. The additivity equation is both intuitively appealing and useful, being able to offer quantitative estimates of the proton affinity by very simple calculation. It is based on the concept of the increment, which in turn describes the influence of a single substituent on the proton affinity. Any substituent behaves as a rule as if the other were non-existent, thus giving rise to the independent substituent approximation (ISA). The performance of the additivity rule of thumb is very good, as evidenced by the average absolute deviation of 1 kcal mol-1. Larger deviations are possible, but they rarely occur, being indicative of a difference in interactions between substituents in the initial neutral base and in the final cationic conjugate acid. Finally, it follows as a corollary of the present analysis that protonation ipso to the CH3 group is never thermodynamically the most favourable site of proton attack in the benzene ring, provided that there is a single unsubstituted carbon atom within the aromatic moiety. The relevance of ipso protonation in persubstituted benzenes is briefly discussed. © 1998 John Wiley & Sons, Ltd.
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    Photoinduced electron transfer and strand cleavage in pyrenyl-DNA complexes and adducts (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 561-565 
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    ISSN: 0894-3230
    Keywords: pyrenyl-DNA complexes and adducts ; photoinduced electron transfer ; strand cleavage ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The fluorescence of pyrenyl residues in complexes with the nucleic acid bases G, C and T, but not A, is strongly quenched by photoinduced electron transfer mechanisms. Site-specifically modified 11-mer oligonucleotide duplexes containing a single modified guanosyl base G* bearing a covalently attached pyrenyl residue were prepared in order to probe for photochemical damage associated with these photoinduced electron transfer reactions. When the pyrenyl residue positioned at G* is photoexcited with 355 nm light, direct strand cleavage is observed at that site with low quantum yield. Frank strand breaks are also observed up to five base pairs away from G*, suggesting that intrastrand migration of a reactive intermediate from base to base is occurring. © 1998 John Wiley & Sons, Ltd.
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    From olefin cyclopropanation to olefin metathesis through catalyst engineering: recent applications of olefin metathesis to fine organic synthesis and to polymer chemistry (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 602-609 
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    ISSN: 0894-3230
    Keywords: olefin metathesis ; olefin cyclopropanation ; catalyst engineering ; fine organic synthesis ; polymer chemistry ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: An overview of the recent synthetic breakthroughs brought about by the discovery of new, functional group tolerant late transition metal (Rh and Ru)-based catalysts is proposed. Whereas dirhodium(II)-based complexes promote only carbene transfer reactions to olefins (i.e. olefin cyclopropanation), a few ruthenium-based catalysts can be engineered and fine tuned so as to mediate either carbene-transfer reactions or olefin metathesis. The different outcome of the reactions can be rationalized by the capability of the metal center to coordinate or not both the carbene and the olefin. This quite simple-minded approach indicates that several available coordination sites at the metal center favors metathesis reactions to the prejudice of olefin cyclopropanation. Examples of recent applications in ring opening metathesis polymerizations and copolymerizations include the formation of postpolymers of polydienes, of carbohydrate-substituted polymers and of telechelic oligomers. Application of the same ruthenium-based catalysts in ring-closing metathesis is illustrated by the formation of crown ether analogs, of unsaturated peptides and amino acids, of β-lactams, and of different bicyclic systems.
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    Imperatives for enzymatic catalysis of isomerization of sugars and sugar phosphates (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 512-518 
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    ISSN: 0894-3230
    Keywords: sugar isomerization ; sugar phosphate isomerization ; enzymatic catalysis ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The relative yields of the products of isomerization of D,L-glyceraldehyde in D2O with intramolecular transfer of a hydride ([1-1H]DHA) and with proton transfer ([1-2H]DHA) were determined by high-resolution 1H NMR analyses. A study of the catalysis of this isomerization by deuteroxide ion, buffer anions and Zn2+ established the following: (1) isomerization with proton and hydride transfer occurs at approximately equal rates in dilute solutions of sodium deuteroxide; (2) Brønsted bases catalyze isomerization with proton transfer in a bimolecular reaction; (3) Zn2+ catalyzes isomerization with hydride transfer in a bimolecular reaction; and (4) Zn2+ and acetate ion react in concert to catalyze isomerization with proton transfer in a termolecular reaction. These results show that the various pathways for isomerization with proton and hydride transfer proceed via transition states of similar energies, so that there is no strong imperative for enzymatic catalysis by any particular reaction mechanism. The relevance of these results to the mechanism of action of xylose isomerase is discussed briefly. © 1998 John Wiley & Sons, Ltd.
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    Structural effects of the Grunwald-Winstein correlations in the solvolysis of some simple tertiary alkyl chlorides (1997)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 717-724 
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    ISSN: 0894-3230
    Keywords: SN1 solvolysis ; carbocation ; Grunwald-Winstein relationship ; solvent effect ; solvation ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: ---The rates of solvolysis in various solvents at 25 °C were determined for five tertiary alkyl chlorides: 2-chloro-2,4,4-trimethylpentane (4), 2-chloro-2,4-dimethylpentane, 2-chloro-2-methylpentane, 1-chloro-1,3,3-trimethylcyclopentane (7) and 1-chloro-1-methylcyclopentane. The rate data were analysed on the basis of the original and extended Grunwald-Winstein-type equation [log(k/k0)=mYCl+c and log(k/k0)=lNT+mYCl+c] and the results were compared with those reported for 2-chloro-2-methylpropane (1) and 2-chloro-2,3,3-trimethylbutane (3). The rate data for 4 in 18 solvents give an excellent correlation with l=0·00±0·02 and m=0·74±0·01. The neopentyl group in 4 more effectively shields the rear-side of the reaction center than the tert-butyl group in 3 that is correlated by l=0·10±0·04 and m=0·81±0·04. The rate ratio between 4 and 1 at 25 °C is 275 in TFE and predicted to increase to 950 in TFA. The previous 4/1 rate ratio of 21 in 80% ethanol evidently underestimates the B-strain effect on the solvolysis rate of 4 by a factor of at least 40. The remote methyl groups in 7 works to increase rear-side shielding without increasing B-strain. The marked difference in the effect of the remote methyl groups between 4 and 7 suggests that the leaving chloride ion in 4 takes a locus that is nearly antiperiplanar to the tert-butyl group. © 1997 John Wiley & Sons, Ltd.
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    Kinetics and mechanism of the aminolysis of p-nitrophenyl N-phenylcarbamates (1997)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 725-730 
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    ISSN: 0894-3230
    Keywords: p-nitrophenyl N-phenylcarbamates ; stepwise mechanism ; rate-limiting breakdown of T± ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: ---Kinetic studies of the reactions of p-nitrophenyl N-phenylcarbamates with benzylamines were carried out in acetonitrile at 25·0 °C. Second-order (k2) and third-order (k3) rate constants were observed for all the Y-substituted carbamates except for Y=m-Cl. The relatively large magnitude of ρX (for X-substituted benzylamines) and ρY together with a positive cross-interaction constant ρXY supports a stepwise mechanism involving rate-limiting breakdown of the zwitterionic tetrahedral intermediate T±. Kinetic isotope effect studies with deuterated benzylamine (XC6H4CH2ND2) indicate that in the base-catalyzed path, k3, rate-limiting deprotonation occurs at the amino group of benzylamine within the T± intermediate. The low δH≠ and δS≠ values for the k3 process are in accord with the proposed mechanism © 1997 John Wiley & Sons, Ltd.
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    Infrared and 1H NMR studies of hydrogen bonding in N-(2-amino-2-deoxy-β-D-glucopyranoside)-N′-carbamoyl-L-amino acid esters (1997)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 747-754 
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    ISSN: 0894-3230
    Keywords: ureido sugars ; amino acids ; 1H NMR ; IR ; H-D exchange ; intramolecular hydrogen bonding ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: ---Ureido 2-deoxy-β-D-glucopyranosides with seven different amino acid ester residues were studied by means of IR and 1H NMR spectroscopy. The H-D exchange rates increase in the order L-Val〈L-Leu〈L-Ala〈Gly for both NH protons; however, the exchange rate at N-1-H (linked to the glucopyranoside) is significantly faster than that at N-3-H (at the amino acid residue). The analysis of IR spectra in the region of the NH stretching vibrations shows, in agreement with other investigations, that the signals at 3454, 3423 and 3355 cm-1 are due to the free and the intramolecular associated NH groups forming a five- and seven-membered ring, respectively. It was found that the C7 associated ring is formed by hydrogen bonding between the N-1-H function and the C=O of the acetyl group at the C-3 position in the glucopyranoside. The N-3-H group is involved in a hydrogen bond with the C=O; function of the ester group protecting the amino acid residue. Furthermore, it was found that the ureido sugar with L-Val exhibits a stronger C7 hydrogen bond than the other amino acid residues. This result conforms with the lowest H-D exchange rate at the N-1 position of this compound. Steric effects resulting in the shielding of this hydrogen bond against OD attack are considered as a reason for this peculiarity. © 1997 John Wiley & Sons, Ltd.
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    Solvent effects on aromatic nucleophilic substitution reactions. Part 7. Determination of the empirical polarity parameter ET(30) for dipolar hydrogen bond acceptor-co-solvent (chloroform or dichloromethane) mixtures. Kinetics of the reactions of halonitrobenzenes with aliphatic amines (1997)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 849-860 
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    ISSN: 0894-3230
    Keywords: solvent effects ; aromatic nucleophilic substitutions ; empirical polarity parameter ; halonitrobenzenes ; aliphaticamines ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Empirical solvent polarity parameters ET(30) were determined by UV/VIS spectroscopy, using Dimroth-Reichardt's betaine dye, as a function of composition, for several binary solvent mixtures [i.e. polar hydrogen bond acceptor (PHBA) solvents+chloroform or dichloromethane]. Each solvent system was analyzed according to its deviations from additivity due to preferential solvation of the chemical probe and also from complicated intermolecular interactions of the mixed solvents. The ET(30) parameter of many of these mixtures has presented synergism. The synergetic effects were more significant for those binary solvent systems in which chloroform is the co-solvent. These results were related to the solvent effects on some aromatic nucleophilic substitution reactions. The kinetics of the reactions between 1-halo-2,4-dinitrobenzenes and primary or secondary aliphatic amines were studied in three solvent systems (PHBA+chloroform) where the synergism for the ET(30) polarity parameter is the rule. In all the aminodehalogenation reactions discussed the formation of the intermediate is the rate-determining step. The kinetic data show a tendency to decrease with decrease in the overall solvation capability of the binary mixture. In general, the reaction rates presented a gradual decrease in the PHBA solvent-rich zone and a large decrease at high co-solvent concentrations. The ET(30) values corresponding to binary dipolar hydrogen bond acceptor-hydrogen bond donor mixtures may be not generally valid for interpreting solvation effects on the reactions under consideration. © 1997 John Wiley & Sons, Ltd.
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    Solvent effect on polar and spectroscopic properties of 2-N-methylamino-5-nitro-6-methylpyridine and 2-N-methylamino-3-nitro-6-methylpyridine (1997)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 835-840 
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    ISSN: 0894-3230
    Keywords: N-methylaminopyridines ; polar properties ; spectroscopic properties ; solvent effect ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Dipole moments of 2-N-methylamino-5-nitro-6-methylpyridine (I) and 2-N-methylamino-3-nitro-6-methylpyridine (II) were determined in solvents of different polarity and basicity. The solvent effect is discussed in terms of the two-parameter (π*-β) Kamlet-Taft expression. The solvent dependences of the energy of intramolecular charge-transfer transition and the frequency of the ν(NH) stretching vibration are in keeping with the dielectric results. It was found that the solvent-induced disturbance in the electronic structure of I is brought about by hydrogen bond interaction with the NH group. The strong intramolecular hydrogen bond in II is stable in weakly basic solvents and the electronic structure is insensitive to the environment. However, in more basic solvents the intramolecular hydrogen bond is partly broken and an N - H· · ·solvent hydrogen bond is formed, bringing about distinct changes in polarity and spectroscopic properties. © 1997 John Wiley & Sons, Ltd.
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    Thiapyrylium salt-sensitized electron transfer reactions of trans-stilbene. Dimerization and oxygenation (1997)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 861-869 
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    ISSN: 0894-3230
    Keywords: thiapyrylium salt ; photoinduced electron transfer ; laser flash photolysis ; trans-stilbene cation radical ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Photoinduced electron transfer reactions of trans-stilbene sensitized by 2,4,6-triphenylthiapyrylium tetrafluoroborate (STPP) were carried out by steady-state and laser flash photolysis techniques in the presence and absence of oxygen in dichloromethane. Rapid dimerization of the trans-stilbene cation radical with its neutral species was observed, as previously observed in 2,4,6-triphenylpyrylium tetrafluoroborate (TPP)-sensitized reactions in dichloromethane. Electrochemical and photophysical properties such as fluorescence quantum yield and T-T absorption spectrum of STPP were also studied, and the results were compared with those for TPP. The properties of STPP as an electron transfer sensitizer are discussed. © 1997 John Wiley & Sons, Ltd.
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    Selective arylation of exocyclic N-position of 2-pyrimidylnitrenium ion photolytically generated from tetrazolo-[1,5-a]pyrimidine in the presence of trifluoroacetic acid (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 478-484 
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    ISSN: 0894-3230
    Keywords: Selective arylation ; exocyclic N-position ; 2-pyrimidylnitrenium ion ; tetrazolo[1,5a]pyrimidine ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Photo-reactions of tetrazolo[1,5-a]pyrimidine (1) with benzene (2a) and substituted benzenes (2b-f) in the presence of trifluoroacetic acid (TFA) gave 2-(2-, 3- and 4-substituted anilino)pyrimidines (3-5) together with 2-aminopyridine (6) and biphenyl (7) or diarylmethanes (8b, 8c and 8e). From the effect of heavy-atom solvent on the reactions, it is reasonable to assume that 3-5 are formed via a singlet species, but 6-8 via a triplet species. The intermediacy of 2-pyrimidylnitrenium ion is consistent with the evidence derived by the above effect, by a Hammett plot with ρ = -2.9 and by effects of solvent nucleophilicity and counter-anions. The selective exocyclic N-arylation giving 3-5 is discussed. © 1998 John Wiley & Sons, Ltd.
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    Kinetics of the hydrolysis of cyclic N-substituted sulfamides: 4-amino-2-cyclohexyl- and 4-amino-2-phenethyl-2,3-dihydro-3-oxo-1,2,5-thiadiazole 1,1-dioxides (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 489-494 
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    ISSN: 0894-3230
    Keywords: Cyclic N-substituted sulfonamides ; 1,2,5-thiadiazole 1,1-dioxide derivatives ; hydrolysis ; kinetics ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The hydrolysis reactions of 4-amino-2-phenethyl- and 4-amino-2-cyclohexyl-2,3-dihydro-3-oxo-1,2,5-thiadiazole 1,1-dioxide (Ia and Ib) were investigated in the range 24-73 °C in buffered aqueous solutions. The observed rate constants (kobs) are independent of pH in the range ca 1-4 pH, but increase with increase in pH above ca 4. A linear log kobs vs pH profile with unit slope is observed from pH ca 4 up to the highest experimental pH (ca 10). The products are the corresponding new compounds: 2-amino-2-[(N-substituted-sulfamoyl)imino]acetic acid salts. The C=N bond of these compounds hydrolyses further, in a slow reaction, to the sulfamide and oxalic acid derivatives. The substrates decompose to the final products without accumulation of the acetic acid derivatives under these experimental conditions. A mechanism is proposed. Rate constants and activation parameters are given for the first reaction step. Owing to steric effects, the reaction rate is higher for the N-phenethyl-substituted derivative than for the 2-cyclohexyl-substituted derivative. © 1998 John Wiley & Sons, Ltd.
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    Rate enhancement of 1,3-dipolar cycloaddition of N-methylindole: the singular role of Grignard reagents (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 455-466 
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    ISSN: 0894-3230
    Keywords: 1,3-dipolar cycloaddition ; N-methylindole ; Grignard reagents ; rate enhancement ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The reactions of 2-N-methylindolyl and 2- and 3-benzo[b]thiophenyl anions with nitrile oxides and nitrilimines were performed in order to investigate whether the use of anionic dipolarophiles modifies a classical 1,3-dipolar cycloaddition. When lithium compounds were used as bases, the heterocyclic anions invariably acted as nucleophilic species; in contrast, when a Grignard reagent was employed, the N-methylindole gave cycloaddition products with an extraordinary rate enhancement. The hypothesis that N-methylindole and ethylmagnesium bromide give an adduct much more reactive than N-methylindole itself was supported by the results of a theoretical investigation. The structure and electron distribution of the adduct were determined by ab initio calculations and compared with those of known Grignard complexes with nitrogen ligands. The performance of different basis sets was tested. The quantum theory of atoms in molecules was used to determine atomic charges and to describe the nature of bonds in terms of the properties of the electron density at the bond critical points. © 1998 John Wiley & Sons, Ltd.
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    The ring inversion of silacycloheptatriene and cycloheptatriene. Comparison of the ‘aromaticity’ of planar and boat conformers estimated by nucleus-independent chemical shift (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 475-477 
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    ISSN: 0894-3230
    Keywords: cycloheptatriene ; silacycloheptatriene ; ring inversion ; aromaticity ; ab initio calculation ; nucleus-independent chemical shift ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Nucleus-independent chemical shifts (NICS) of boat and planar structure of silepins (1) and cycloheptatriene (2) were calculated at the HF/6-31+G* level for the Becke3LYP/6-31G* optimized geometries in order to investigate the correlation between the inversion barrier and aromatic nature in the boat and planar forms of 1 and 2. The NICS of unsubstituted silepin 1a indicated that there is almost no stabilization in the boat form (NICS, -0.5) and that the planar form (NICS, 3.5) is relatively destabilized in comparison with the boat form. On the other hand, the NICS of 2 showed that there is considerable homoconjugative stabilization in the boat form (NICS, -4.2) and destabilization in the planar form (NICS, 8.2). In addition to the geometrical effects resulting from angle strain, these electronic effects are considered to affect the energy barriers for ring inversion, which have been calculated to be 0.9 kcal mol-1 for 1a and 5.2 kcal mol-1 for 2. © 1998 John Wiley & Sons, Ltd.
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    Hydrogen-bonding-based thermochromic phenol-amine complexes (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 737-742 
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    ISSN: 0894-3230
    Keywords: thermochromism ; proton transfer ; hydrogen bond ; phenol ; amine ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Variable-temperature UV-vis, 13C NMR and IR studies showed that proton-transferred complexes were formed between phenols and amines in apolar solvents at low temperature. Upon cooling a solution of p-nitrophenol and diisopropylamine in toluene, the colour of the solution changed from colourless to yellow. This thermochromism was ascribed to the proton transfer in the hydrogen-bonding complex. Under UV-vis conditions, butylamine and imidazole also caused similar thermochromism upon complexation with p-nitrophenol, while triethylamine, quinuclidine and pyridine did not. The thermochromic behaviour was particularly dependent on the stoichiometry of the amine and the phenol: a solution of 3,3′-dibromo-5,5′-dinitro-2,2′-biphenyldiol and diisopropylamine with a molar ratio of 1:1 showed no thermochromism, while solutions with 1:2 or higher ratios showed thermochromism, indicating that excess amine is required to obtain the proton-transferred species. These results revealed that the proton-transferred species forms in apolar solvents at low temperature if an appropriate hydrogen-bonding network between the phenol and the amine can stabilize it. © 1998 John Wiley & Sons, Ltd.
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    Double fragmentation in cation radicals. An example in the NADH analogues series (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 774-780 
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    ISSN: 0894-3230
    Keywords: double fragmentation ; cation radicals ; NADH analogues ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The cation radical of 9-tert-butyl-N-methylacridan, generated electrochemically or photochemically, offers, in the presence of strong bases, a remarkable example of a double fragmentation. Whereas in acidic or weakly basic media the tert-butyl radical is cleaved with concomitant formation of the methylacridinium cation, the presence of a strong base triggers the cleavage of both the methyl group borne by the nitrogen atom and the tert-butyl group on C-9 leading to acridine, formaldehyde and the tert-butyl anion, even though methylacridinium cation is stable under these conditions. The origin of this unprecedented behavior resides in the prior deprotonation of the methyl group borne by the nitrogen atom which outruns the usual deprotonation at the 9-carbon because this is slowed by the steric hindrance due to the presence of the tert-butyl group. © 1998 John Wiley & Sons, Ltd.
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    Effect of sulfuric acid on reactions of peroxymonosulfate ion with sulfides and thiol esters (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 803-808 
    Details
    ISSN: 0894-3230
    Keywords: peroxymonosulfate ion ; oxidation ; sulfuric acid ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Peroxymonosulfate ion, HSO5-, as Oxone in aqueous H2SO4, oxidizes sulfides [MeSC6H4X(p), X = Me, H, NO2] to sulfoxides and converts aryl thiobenzoates [PhCOSC6H4X(p), X = Me, H] and thiol phosphorus(V) esters [Ph2POSPh, (EtO)2POSPh, Ph2POSEt, Ph(EtO)POSEt] into the acids and sulfonate ions. Second-order rate constants increase with increasing concentration of H2SO4 (10-53 wt%), owing to the high polarity of the medium rather than to acid catalysis. The rate increases fit the Grunwald-Winstein equation based on YOTs solvent parameters derived from rate effects on SN1 solvolyses. © 1998 John Wiley & Sons, Ltd.
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    Cation radical cycloaddition polymerization: Diels-Alder copolymerization (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 825-830 
    Details
    ISSN: 0894-3230
    Keywords: cation radical cycloaddition polymerization ; Diels-Alder copolymerization ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The Diels-Alder cycloaddition copolymerization of a bis(diene) with ionizable bis(dienophiles) via a cation radical mechanism has been accomplished using tris(4-bromophenyl)aminium hexachloroantimonate as a catalyst in dichloromethane solvent. The reactions occur at 0°C and yield Diels-Alder polymers of MW up to ca. 10,000 and a polydispersity index ca. 2. © 1998 John Wiley & Sons, Ltd.
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    Solvent effects on aromatic nucleophilic substitutions. Part 6. Kinetics of the reaction of 1-chloro-2, 4-dinitrobenzene with piperidine in binary solvent mixturesPresented in part at the XX Congreso Argentino de Química, Córdoba, Argentina 1994. (1996)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 9 (1996), S. 459-470 
    Details
    ISSN: 0894-3230
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The kinetics of the reaction between 1-chloro-2,4-dinitrobenzene and piperidine was studied in several completely non-aqueous binary solvent mixtures where the preferential solvation is the rule at 15, 25 and 40°C. The reaction was chosen as the simplest example of aromatic nucleophilic substitutions (ANS). For (aprotic solvent + aprotic co-solvent) binary systems the co-solvent was toluene, and the rest of the solvents used were selected with different structural characteristics and an extensive range of polarity. In this kind of mixture a property of mixed binary solvents would be defined by means of ET(30) values and the solvent effects on this simple model of ANS reactions are similar to those of aprotic pure solvents, especially if hydrogen-bond donor solvent mixtures are excluded from the analysis. For (aprotic solvent + protic co-solvent) binary systems the co-solvent used was methanol. The presence of a protic solvent in the mixture strongly determines the solvent effects on the reaction. In this type of binary mixture, the chemical probe under consideration may not be generally valid to interpret solvation effects. Additionally, empirical solvent polarity parameters ET(30) were determined UV-VIS spectrophotometrically for some pure aprotic solvents and, as a function of the composition, for (dimethylformamide + toluene), (toluene + methanol) and (1,1,1-trichloroethane + methanol) at 15 and 40°C, with the purpose of extending the studies on the empirical polarity indices in binary solvent mixtures to the thermo-solvatochromic area.
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    Kinetic study of base-promoted elimination reactions of some 1,1,1-trihalo-2,2-bis(dimethoxyphenyl)ethanes in alcoholic solutions (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 54-58 
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    ISSN: 0894-3230
    Keywords: 1,1,1-trihalo-2,2-bis(dimethoxyphenyl)ethanes ; base-promoted elimination ; reaction mechanism ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The base-promoted elimination reactions of 1,1,1-trichloro-2,2-bis(dimethoxyphenyl)ethanes were investigated. The bis(3,4-dimethoxyphenyl)ethane was found to be more reactive than the bis(2,5-dimethoxyphenyl)ethane and the latter more reactive than the bis(2,4-dimethoxyphenyl)ethane. Kinetic data relative to 1,1,1-trihalo(chloro or bromo)-2,2-bis(3,4-dimethoxyphenyl)ethanes show that the tribromo reacts faster than trichloro derivative and that the reactions are general-base promoted with Brønsted β values of about 0.6. A kinetic isotope effect, with kH/kD ratio ranging from 3.5 to 5.7, for the base-promoted elimination reaction of 1,1,1-trichloro-2,2-bis(3,4-dimethoxyphenyl)ethane was found. Tunneling occurs for methoxide and ethoxide ion-promoted eliminations. Activation parameters for alkoxy-promoted elimination show a similar trend for chloride and bromide derivatives. The data collected seem to confirm that there is contiguity between E1cBirr and E2 mechanisms. © 1998 John Wiley & Sons, Ltd.
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    Kinetic and thermodynamic study of 2-bromo-2-methylbutane, 2-chloro-2-methylbutane and 3-chloro-3-methylpentane in diols (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 36-40 
    Details
    ISSN: 0894-3230
    Keywords: haloalkanes ; diols ; kinetics ; thermodynamics ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Rate constants, k, were determined for the solvolytic reactions of 2-bromo-2-methylbutane, 2-chloro-2-methylbutane and 3-chloro-3-methylpentane in 10 pure diols at 298.15 K. The Gibbs energies of activation were calculated in order to study the influence of the solvent and to provide mechanistic information about the activation process. By combining these values with a set of infinite dilution activity coefficients, γ∞, the transfer Gibbs energies of the reactants (initial state) and of the activated complex (transition state) were obtained, which allowed the solvent effects on both states to be quantitatively analysed. The γ∞ values were calculated using the UNIFAC group contribution method. The modified Flory-Huggins equation was used in the combinatorial term. The interaction parameters were taken from the specific UNIFAC γ∞ interaction parameter table, except for the γ∞ values of 2-chloro-2-methylbutane and 3-chloro-3-methylpentane in diethylene glycol and triethylene glycol, which were obtained using the γ∞ UNIFAC-VLE interaction parameter tables. For comparative purposes all the other infinite dilution activity coefficients of the three solutes in the diols were also calculated using the UNIFAC-VLE parameter tables. © 1998 John Wiley & Sons, Ltd.
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    Inhibitory effect of DNA structure on the efficiency of reaction of guanosine moieties with a nitrenium ion (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 71-76 
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    ISSN: 0894-3230
    Keywords: guanosine ; nitrenium ; DNA structure ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The N-acetyl-N-(2-fluorenyl)nitrenium ion (2a) reacts very efficiently with monomeric 2′-deoxyguanosine (d-G) to form a C-8 adduct, N-(2′-deoxyguanosin-8-yl)-2-acetylaminofluorene (6), in an aqueous environment, with a selectivity ratio, kd-G/ks, of 13.1 × 103 M-1 at 0°C and 4.8 × 103 M-1 at 30°C. The reactivity of the self-complementary oligomer d-ATGCAT with 2a can be separated into components due to the single-stranded (SS) and double-stranded (DS) forms. Within the error limits of the measurements kSS/kd-G ≈ 0.27 and kDS/kd-G ≈ 0. Another measure of the reactivity of d-G moieties in the DNA double helix can be obtained from measurements with the circular super-coiled plasmid pUC19. This plasmid provides an upper limit for kpUC19/kd-G of 0.02, where kpUC19 is the average trapping rate constant per d-G moiety in pUC19. The strong inhibition of the trapping reaction caused by the tertiary structure of the DNA double helix may be responsible for the change in product distribution of 2a-d-G adducts found from reaction with d-G, and denatured DNA (exclusive C-8 adduct, 6) and native DNA [5-20% N-2 adduct,3-(2′-deoxyguanosin-N2-yl)-2-acetylaminofluorene, 7]. © 1998 John Wiley & Sons, Ltd.
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    Effects of urea, Na+ and Li+ ions on the kinetics and mechanism of intramolecular general base-catalyzed glycolysm of ionized phenyl salicylate in ethane-1,2-diol-acetonitrile solvents at a constant water concentration (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 109-114 
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    ISSN: 0894-3230
    Keywords: phenyl salicylate ; ethane-1,2-diol ; urea ; sodium salt ; lithium salt ; transesterification ; kinetics ; intramolecular general base catalysis ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Pseudo-first-order rate constants (k1) for the reaction of ethane-1,2-diol (DOL) with ionized phenyl salicylate (PS-), obtained in mixed DOL-CH3CN solvent at constant [H2O] and [NaOH], obey the relationship k1  =  α[DOL]T/(1 + 2KA[DOL]T), where α is the apparent second-order rate constant, KA is the association constant for the dimerization of DOL and [DOL]T is the total concentration of DOL. The values of KA, in the presence of Na+ ions, decrease with increase in [H2O]. Lithium ions cause almost complete depolymerization of polymeric DOL (i.e. KA ≈ 0) under the experimental conditions imposed. The effect of 0.5 M urea on the structural behavior of the mixed solvent is kinetically insignificant. © 1998 John Wiley & Sons, Ltd.
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    Effects of oligooxyethylene linkage on intramolecular [2 + 2] photocycloaddition of styrene derivatives (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 79-83 
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    ISSN: 0894-3230
    Keywords: [2 + 2] ; photocycloaddition ; quantum yield ; oligooxyehylene linkage ; cyclophane ; styrene derivatives ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: There are two possible explanations for the high yields in the intramolecular [2 + 2] photocycloaddition of compounds 3a-d in which two styrene moieties are bridged by an oligooxyethylene linkage: one is the electronic effects of phenoxy oxygen atoms at the para-position of the vinyl group and the other is the steric effects due to the flexibility of oligooxyethylene linkages. In order to clarify the contributions of the two effects, a study was made of the photoreaction of similar compounds in which oxygen atoms are present only at the ends of the chain, and therefore only the electronic effects are involved. In these compounds, the quantum yields of the photocycloaddition were found to be low. Hence the high yields in the photocycloaddition of 3a-d are concluded to be ascribable to the flexibility of oligooxyethylene linkages. © 1998 John Wiley & Sons, Ltd.
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    Improved method for estimating the ZFS parameter D for delocalized biradicals (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 819-824 
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    ISSN: 0894-3230
    Keywords: zero field splitting parameter, D ; organic biradicals ; triplets ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: An ‘average’ approximation for calculating the zero-field splitting parameter, D, gives reasonable results when used to calculate D values for non-disjoint delocalized organic biradicals. When used to calculate disjoint localized organic biradicals the D values are approximately half the experimental values. © 1998 John Wiley & Sons, Ltd.
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    Correlation analysis of UV spectral data of some phenylhydrazones and semicarbazones by the dual-parameter equation. Observation of three types of behaviors of the λmax values induced by the polar effects of substituents (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 809-818 
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    ISSN: 0894-3230
    Keywords: UV spectra ; dual-parameter equation ; correlation analysis ; polar effect ; spin-delocalization effect ; p-Y-substituted benzaldehyde phenylhydrazones ; p-Y-substituted benzaldehyde 2,4-dinitrophenylhydrazones ; p-Y-substituted benzaldehyde 4-nitrophenylhydrazones ; p-Y-substituted benzaldehyde N-phenylsemicarbazones ; p-Y-substituted benzaldehyde semicarbazones ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
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    Notes: The K-band λmax values of the UV spectra of five types of p-Y-benzaldehyde derivatives, namely phenylhydrazones (1-Ys), 2,4-dinitrophenylhydrazones (2-Ys), 4-nitrophenylhydrazones (3-Ys), semicarbazones (4-Ys) and N-phenylsemicarbazones (5-Ys), were measured and the corresponding wavenumbers (νmax) calculated. Correlation analyses of the νmax values by the dual-parameter equation show that the νmax values of these compounds are affected, albeit to different degrees, by both spin-delocalization effects (σ·) and polar effects (σx) of the substituents. The spin-delocalization effects of substituents always facilitate bathochromic shifts of the λmax values (ρ· negative). However, polar effects of the substituents on these λmax data depend on the mode of polarization of these hydrazone or semicarbazone molecules, i.e. electron-pair acceptor substituents (Ys) facilitate bathochromic shifts for 1-Ys (ρx negative), but hypsochromic shifts for 2-Ys and 3-Ys (ρx positive), and induce very small shifts for 4-Ys and 5-Ys. As expected, donor Ys facilitate hypsochromic shifts for 1-Ys and bathochromic shifts for 2-Ys and 3-Ys and induce very small shifts for 4-Ys and 5-Ys. Possibly, a higher degree of polarization of the substrate molecule would demand a higher degree of polar assistance from the substituents at the transition states. © 1998 John Wiley & Sons, Ltd.
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    Solvatochromism of dyes. Part III. Solvatochromism of merocyanines in some binary mixtures of solvents.SA-SAB-SB, a new model of solvatochromismFor Part II, see Ref. 1. (1997)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 647-661 
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    ISSN: 0894-3230
    Keywords: merocyanines ; solvatochromism ; solvent polarity ; VBHB model ; SA-SAB-SB model ; binary solvent mixtures ; solvation equilibria ; spectra simulation ; pyridinium betaines ; 7H-indolo[1,2-a]quinolinium betaines ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: ---A quantitative model of solvatochromism in a binary solvent system is presented. Although it is derived for merocyanine and betaine dyes, it explains a majority of known examples of solvatochromism in binary solvents. The model provides an estimation of equilibrium constants between particular types of solvates present therein. UV-VIS absorption spectra of solvated species can be simulated. They perfectly fit the experimental data. The model proposed describes the internal solvent picture from the solute point of view, which differs from other known models and may be useful for studying the structure of liquids © 1997 John Wiley & Sons, Ltd.
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    Comparison of ΔG°Acid (gas phase) and kinetic acidities measured in methanolic sodium methoxide (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 614-617 
    Details
    ISSN: 0894-3230
    Keywords: ΔG°Acid (gas phase) ; kinetic acidities ; methanolic sodium methoxide ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Hydron exchange rates, kexc (M-1s-1), using methanolic sodium methoxide were compared with ΔG°Acid, (kcal mol-1) (gas phase) for 9-phenylfluorene, C6H5CH(CF3)2, m-CF3C6H4CH(CF3)2, p-CF3C6H4CHClCF3, m-CF3C6H4CHClCF3, 3,5-(CF3)2C6H3CHClCF3, fluorene and C6F5H. There is a good linear correlation for p-CF3C6H4CHClCF3, m-CF3C6H4CHClCF3 and 3,5-(CF3)2C6H3CHClCF3, with the others falling off the line. The fluorinated benzyl compounds and pentafluorobenzene have near-unity isotope effects and therefore differ from the fluorenyl compounds. Although the acidity and the exchange rates for three of the compounds [9-phenylfluorene, C6H5CH(CF3)2 and p-CF3C6H4CHClCF3] are similar, the important proton-transfer step to form a hydrogen-bonded carbanion intermediate and the subsequent breaking of that weak bond to form a free carbanion in methanol differ significantly for the fluoernyl compound compared with the two fluorinated benzylic compounds. © 1998 John Wiley & Sons, Ltd.
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    Intramolecular thermal (4σ + 2π) dyotropy: primary 2H kinetic isotope effects. Experimental limiting barrierparameters for quantum tunnelling in 2H transfer processes and mechanistically significant substituent effects (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 879-886 
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    ISSN: 0894-3230
    Keywords: intramolecular thermal (4σ + 2π) dyotropy ; kinetic isotope effects ; limiting barrier parameters ; quantum tunnelling ; 2H transfer processes ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Primary deuterium kinetic isotope effects (PDKIE) in parazoline-annelated syn-sesquinorbornenes exhibiting irreversible intramolecular (4σ + 2π) thermal dyotropy reveal unambiguous evidence for a tunnelling contribution to the kinetics in one instance but not for a close analogue. For analogous dyotropy of a cyclohexadiene-annelated syn-sesquinorbornene, the tunnelling components of the kinetic behaviour is small by comparison. The H atom traverse between alternative loci for the pyrazolines, deduced from x-ray and neutron diffraction data, is in agreement with approximate barrier parameters obtained by fitting of the PDKIE data to the Bell equation; barrier penetration is 3.22 kcal below the computed barrier corrected for the tunnelling contribution. The relative kinetic effect of systematic variation of the π-donor/acceptor groups on aryl ring substituents at C and N in the pyrazoline ring is consistent with a pericyclic process for dyotropy of these compounds, but not with rearrangement mediated by biradicals resulting from single H atom transfer in the rate-limiting step. Computer modelling of the transition state for dyotropy of these compounds is also consistent with a thermal, orbital symmetry conserved pericyclic reaction. Copyright © 1998 John Wiley & Sons, Ltd.
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    Unusual reactivity of highly strained cyclophanesDedicated to Prof. Dr Maitland Jones, Jr, on the occasion of his 60th birthday (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 362-376 
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    ISSN: 0894-3230
    Keywords: highly strained cyclophanes ; reactivity ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: An essential feature of the concept of aromaticity has been the stability and lack of reactivity of aromatics relative to that of other unsaturated compounds. Contrary to this general experience, high and unusual reactivity is encountered when simple, monocyclic benzene rings are bent by short bridges into a boat-shaped conformation, as is the case in small [n] paracyclophanes (n ≤ 8) and [n]metacyclophanes (n ≤ 7). This is illustrated, mostly with examples taken from the authors' own work, for thermal and photochemical behavior and reactions with electrophiles, nucleophiles and dienophiles. © 1998 John Wiley & Sons, Ltd.
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    Heterogeneous catalysis of organic reactions (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 356-361 
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    ISSN: 0894-3230
    Keywords: heterogeneous catalysis ; organic reactions ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: A decade's worth of work is reviewed: building on the demonstrated prowess of heterogeneous catalysis in industrial organic chemistry, the author's laboratory devised efficient catalysts for a number of organic reactions, based on aluminosilicates such as clays and zeolites. This review also spells out, at a time when creative research is overshadowed by imitative research, some of the methodological musts that also happen to characterize physical organic chemistry: the devising of experiments so that accurate numerical data can be obtained; the importance of remote, interdisciplinary connections; the need for estrangement from stereotypic preconceptions that may obscure the true explanations for the phenomena; and the over-riding need for concerning oneself only, or at least predominantly, with the important facts that nature tells, if one cares to listen. © 1998 John Wiley & Sons, Ltd.
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    Determination of complex stabilities with nearly insoluble host molecules. Part II. Complexation of alkali and alkaline earth metal cations with dibenzo crown ethers in aqueous solution (1997)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 781-785 
    Details
    ISSN: 0894-3230
    Keywords: complex stabilities ; alkali metals ; alkaline earth metals ; dibenzo crown ethers ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: ---An experimental method was developed to determine the stability constants with nearly insoluble ligands in homogeneous solution. This method was tested using dibenzo crown ethers, which have a very low solubility in aqueous solution. The stability constants for the complexation of alkali and alkaline earth metal cations with different dibenzo crown ethers were determined in aqueous solution. Owing to the complex formation the total concentration of the ligand in solution increases. Dibenzo crown ethers absorb in the ultraviolet spectral range, hence the increase in the ligand concentration can be easily detected. Without the knowledge of the molar absorptivities of the ligands and of the corresponding complexes and of the solubilities, the stabilities of the complexes formed can be calculated under certain assumptions. To verify these assumptions, the solubilities of the dibenzo crown ethers and the molar absorptivities were determined. The kinetics of the solubilization process of the ligands was followed by spectrophotometric measurements. © 1997 John Wiley & Sons, Ltd.
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    Tandem energy transfer-electron transfer in the photosensitized alkylation of α,β-unsaturated ketones (1997)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 777-780 
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    ISSN: 0894-3230
    Keywords: tandem energy transfer-electron transfer ; photosensitized alkylation ; α,β-Unsaturated ketones ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: -α,β-Unsaturated ketones are not conveniently alkylated by radicals generated from tetraalkylstannanes via photoinduced electron transfer (PET), either by direct irradiation or when a singlet sensitizer (an aromatic nitrile, a pyrilium salt) is used. However, the procedure is successful with tetramethyl pyromellitate (TMPM) as the sensitizer. TMPM is promoted to the triplet state by energy transfer from the unsaturated ketones and then sensitizes the cleavage of alkylstannanes by electron transfer. The alkyl radicals thus formed finally add to the unsaturated ketones, giving the β-alkyl derivatives. © 1997 John Wiley & Sons, Ltd.
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    Kinetic study of the oxidation of propan-1-ol by alkaline hexacyanoferrate(III) catalysed by ruthenium trichloride (1997)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 662-668 
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    ISSN: 0894-3230
    Keywords: propan-1-ol oxidation ; alkaline hexacyanoferrate(III) ; ruthenium trichloride catalyst ; kinetic ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: ---The oxidation kinetics of propan-1-ol by alkaline hexacyanoferrate(III) catalyzed by ruthenium trichloride were studied spectrophotometrically. The initial rate method was used for kinetic analysis. The reaction rate shows a fractional order in [oxidant] and [substrate] and a first-order dependence on [RuCl3]. The dependence on [OH-] is complicated. A reaction mechanism involving two active catalytic species is proposed. Each one of these species forms an intermediate complex with the substrate. The attack of these complexes by hexacyanoferrate(III) in rate-determining step produces a radical species which is further oxidized in the subsequent step. © 1997 John Wiley & Sons, Ltd.
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    Solvatochronism and preferential solvation of aryliminomethylpyridinium iodines in binary mixtures (1997)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 731-736 
    Details
    ISSN: 0894-3230
    Keywords: solvatochromism ; preferential solvation ; aryliminomethylpyridinium iodides ; binary mixtures ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: ---Studies on the solvatochromic behaviour of N-methyl-4- and N-methyl-2-[(4-dimethylaminophenyl)iminomethyl]pyridinium iodide dyes in a variety of solvents and the preferential solvation of the former dye in binary mixtures of protic and non-protic solvents are presented and interpreted in terms of solute-solvent and solvent-solvent interactions. © 1997 John Wiley & Sons, Ltd.
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    Are phosphatetrahedrane and diphosphatetrahedrane phosphorus or carbon bases? (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 678-683 
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    ISSN: 0894-3230
    Keywords: phosphatetrahedrane ; diphosphatetrahedrane ; phosphorus bases ; carbon bases ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The protonation processes of phosphotetrahedrane (PTH) and diphosphatetrahedrane (DPTH) were studied using density functional theory approaches. The geometries of the neutrals and the protonated species were optimized at the B3LYP/6- 31G(d) level and the final energies were obtained using the B3LYP/6- 311+G(3df,2p) level of theory. Both PTH and DPTH are tetrahedral compounds which behave as carbon bases in the gas phase. In contrast with what was found for other phosphorus-containing cage structures such as tetraphosphacubane, the nature of the basic center does not change when the hydrogen atoms are substituted by bulky substituents. This is a consequence of the fact that both phosphorus and carbon protonation processes lead to C - P bond fission, so that the repulsive interactions between the susbtituents are not significantly different in carbon- or phosphorus-protonated structures. Protonation of DPTH at the P - P bond yields a local minimum with a P - H - P linkage similar to that described before for the most stable protonated form of P4. This seems to indicate that the existence of P - P linkages in strained structures clearly favors the formation of non-classical protonated species where the proton bridges the two phosphorus atoms. © 1998 John Wiley & Sons, Ltd.
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    Novel aspects of carbonyl oxide chemistry (1997)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 787-796 
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    ISSN: 0894-3230
    Keywords: carbonyl oxide ; ozone ; structure and reactivity ; oxygen atom transfer ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The structural and chemical properties of 1,3-dipolar peroxidic species (X - O - O) such as ozone (X=O) and carbonyl oxides (X=R2C) depend significantly on the nature of X. Although cyclic O3 is thermodynamically unstable, dioxiranes, cyclic isomers of carbonyl oxides, are isolable and have been fully characterized. In contrast to the well known electrophilic nature of ozone, carbonyl oxides usually act as a nucleophilic oxygen transfer agent, but their reactivity could be altered by substituents. It is expected that the chemistry of R2C - O2 species will provide a good starting point for a better understanding of dioxygen complexes (X - O2) with various Xs. Interesting aspects of carbonyl oxide chemistry are discussed, including methods of generation, cyclization to dioxiranes and the nature of their oxygen transfer activities. © 1997 John Wiley & Sons, Ltd.
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    X-ray and vibrational studies of the complex between N-tert-butoxycarbonyl-L-phenylalanine and pyridine. Isotope effect (1997)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 825-834 
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    ISSN: 0894-3230
    Keywords: N-tert-butoxycarbonyl-L-phenlylalanine-pyridine complex ; isotope effect ; x-ray structure ; vibrational spectroscopy ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The x-ray structure of the complex between N-tert-butoxycarbonyl-L-phenylalanine and pyridine shows that the crystals are held together by short hydrogen bonds between the OH group and the N atom of the pyridine ring [RO(H)· · ·N=2·574(3) Å]. Deuteration of the OH and NH groups results in a small expansion of the unit cell associated with an elongation of the O· · ·N distance [RO(D)· · ·N=2·610(3) Å]. The infrared spectra show very broad stretching protonic bands in the range 2750-600 cm-1. The isotopic ratio ν(OH· · ·H)/ν(OD· · ·N) of nearly unity suggests a double minimum potential with a low barrier for the hydrogen bridge. The Raman and near-infrared spectra are discussed. The perturbation of the fundamental modes and of the overtones of pyridine suggests that the complex is not of the proton transfer type. The broad absorptions of the hydrogen bridge disappear in the near-infrared region. The perturbation of the pyridine overtones is discussed. © 1997 John Wiley & Sons, Ltd.
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    Solid-state NMR and x-ray diffraction studies of ionic complex of 1,8-bis(dimethylamino)naphthalene (DMAN) with picrolonic acid (1997)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 814-824 
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    ISSN: 0894-3230
    Keywords: 13C and 1H magic angle spinning NMR ; x-ray diffraction ; 1,8-bis(dimethylamino)naphthalene ; picrolonic acid ; multicentre hydrogen bonding ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: An ionic complex of 1,8-bis(dimethylamino)naphthalene with picrolonic acid was studied by 13C and 1H magic angle spinning and x-ray diffraction. Structural and spectroscopic features of this compound were investigated. The detailed x-ray structure of this complex is described. A multicentre model of hydrogen bonding in proton sponges {[Me2N - H· · ·NMe2]+· · ·Xδ-} is proposed and the influence of weak intermolecular intractions with the nearest electronegative atom in crystal lattice of a proton sponge on the strong intramolecular [N - H· · ·N]+ hydrogen bonding is demonstrated. It appears that weak interactions of electronegative atoms with the nearest methyl hydrogen atoms can influence the localization of the proton in the intramolecular [N - H· · ·N]+ hydrogen bridge. © 1997 John Wiley & Sons, Ltd.
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    How to get wrong results from good experimental data: a survey of incorrect applications of regression (1997)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 797-813 
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    ISSN: 0894-3230
    Keywords: statistics ; regression ; correlation analysis ; incorrect data processing ; least-squares method ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Examples are given from older and more recent literature (kinetics, ionization equilibria, complex formation in solution, dipole moment determination, thermochemistry, resonance energies, NMR shifts, photoelectron spectroscopy) where experimental data were processed in an incorrect way from the point of view of statistics. The results were more or less biased, sometimes completely wrong. Corrected procedures, based entirely on the least-squares method, are reported; in several cases methods are proposed. Some hints are given as to how these mistakes can be avoided, how they can be revealed in the literature and how the literature data can be recalculated: the last task is the most difficult. © 1997 John Wiley & Sons, Ltd.
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    Solvolysis of 1-alkyl-1-chloro-1-(4-methyl)phenylmethanes. Nucleophilic solvent intervention and extended YBnCl scale (1997)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 879-884 
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    ISSN: 0894-3230
    Keywords: solvolysis ; correlation analysis ; azide salt effect ; β-deuterium kinetic isotope effect ; YBnCl ; 1-alkyl-1-chloro-1-(4-methyl)phenylmethanes ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The solvolysis of 1-alkyl-1-chloro-1-(4-methyl)phenylmethanes (4a-d) in aqueous acetone, aqueous ethanol, aqueous methanol and ethanol-trifluoroethanol was studied. Grunwald-Winstein-type correlation analysis using the YBnCl scale suggests significant nucleophilic solvent intervention in the case of 1-chloro-1-(4-methyl)phenylethane (4a). Increasing bulkiness of the 1-alkyl substituent from methyl (4a) to ethyl (4b), to isopropyl (4c) and to tert-butyl (4d) resulted in a gradual change to limiting SN1 mechanisms. The observed excellent linear correlations with YBnCl and the good solubility in high-water-containing binary solvents made 4d a suitable reference standard for deriving more YBnCl values. A positive azide salt effect was realized in the solvolysis of 4a but not 4d. A small decrease in the β-deuterium kinetic isotope effect from 4a to 1-chloro-1-(4-methoxyphenyl)propane (5) suggested the presence of additional stabilization of the benzylic cationic transition state. Howver, no relationship between k(CH3)/k(CD3) and the solvent effect was found. The superiority of employing the YBnCl scale over the combination of YCl and I scales in the mehanistic study was observed. © 1997 John Wiley & Sons, Ltd.
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    Improved method of calculating group polarization effects: constants σα (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 485-488 
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    ISSN: 0894-3230
    Keywords: group polarization effects ; polarizability countants ; ab initio ; Sadlej's basis set ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: An improved model is presented for ab initio calculations of the polarizability constants σα using Sadlej's basis set. A new set of constants was calculated for common groups and these were compared with previous 3-21G and 3-21G* calculations and with some experimental equilibrium constants in the gas phase. For the time being, the calculations seem to be the most general approach for obtaining these constants whereas the experimental possibilities are limited in scope. © 1998 John Wiley & Sons, Ltd.
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    1:1 Crystal complex of 2′,6′-dimethoxyflavone and 2,6-dimethoxybenzoic acid. Relationship of pKa to x-ray hydrogen bond data (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 751-755 
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    ISSN: 0894-3230
    Keywords: 2′,6′-dimethoxyflavone-2,6-dimethoxybenzoic acid complex ; crystal structure ; pKa values ; hydrogen bonds ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The crystal structure of the 2′,6′-dimethoxyflavone-2,6-dimethoxybenzoic acid complex was determined. Owing steric hindrance of the methoxy groups, the two H-bonded molecules are twisted. Earlier results were used to establish a relationship between the pKa values of different acids and their hydrogen bond distances in complexes with 2′,6′-dimethoxyflavone. © 1998 John Wiley & Sons, Ltd.
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    Stability and nitrosation efficiency of substituted N-methyl-N-nitrosobenzenesulfonamides (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 756-760 
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    ISSN: 0894-3230
    Keywords: N-methyl-N-nitrosobenzenesulfonamides ; stability ; nitrosation efficiency ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: A series of substituted N-methyl-N-nitrosobenzenesulfonamides [2,4,6-(CH3)3, 4-CH3O, 4-CH3, 4-Cl and 4-NO2] were synthesized. All of them transfer their nitroso group to N-methylaniline in a quantitative manner, the more reactive being those substituted with electron-withdrawing groups, thus resembling some of the known alkyl nitrites. Studies of their acid denitrosation and base-catalysed hydrolysis demonstrated that the nitrosobenzenesulfonamides are fairly stable in aqueous media between pH 2 and 11. Their relative stability in aqueous media together with their ability to transfer the nitroso group to nucleophiles suggest their use as excellent alternatives to alkyl nitrites in both neutral and basic media. © 1998 John Wiley & Sons, Ltd.
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    Conformational analysis of substituted 5,6,7,8-tetrahydrodibenzo[a,c]cyclo-octene: comparison of 1H NMR, molecular mechanics and semiempirical methods (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 781-786 
    Details
    ISSN: 0894-3230
    Keywords: eight membered rings ; conformational analysis ; molecular mechanics ; semiempirical methods ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The 6-Mono- and trans-6,7-di-substituted derivatives of 5,6,7,8-tetrahydrodibenzo[a,c]cyclo-octene exist in solution in two different conformations. The experimental ΔGo values obtained from 1H NMR spectra are compared with the ΔEsteric and ΔG values calculated by molecular mechanics and semiempirical quantum mechanics methods respectively. The experimental ΔGo values are reproduced better by the ΔEsteric values. The semiempirical methods predict the order of stability of the conformers correctly except for two cases in the PM3 and AM1 methods, but the estimated values are far from the experimental ones. The standard deviation between the experimental ΔGo values and those calculated by semiempirical methods is less for AM1 and MNDO and greater for PM3 and MINDO/3. The conformational space of the flexible side chains in monoacetoxy 1d, monohydroxy 1e, trans-diacid 2d, trans-dimethyl ester 2e and trans-bis-hydroxymethyl 2f searched by molecular mechanics is not reproduced completely by semiempirical methods. © 1998 John Wiley & Sons, Ltd.
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    Effect of solvent composition on the acidity of phenols in dimethyl sulfoxide-water mixtures (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 847-852 
    Details
    ISSN: 0894-3230
    Keywords: phenols ; acidity ; solvent composition ; supersolvent ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The effect of solvent composition on the acidity of substituted phenols in DMSO-H2O systems was examined. It is shown that in DMSO-H2O mixtures with up to 35 mol% DMSO there is a linear relationship between the pKa values of monosubstituted phenols in the binary solvent mixtures of different compositions and the corresponding pKa values in water. This leads to an equation relating the slope and intercept terms, pKa(DMSO-H2O) = slope[pKa(H2O)] + intercept. The basis of this relationship in the Kirkwood-Westheimer equation is examined. The significance of a possible ‘supersolvent’ is discussed. The applicability of the relationships derived herein to other rate-equilibria relationships and solvent mixtures is considered. Copyright © 1998 John Wiley & Sons, Ltd.
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    Dipole moments of polyenic oligomeric systems. Part I. A one-dimensional molecular wire model (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 853-856 
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    ISSN: 0894-3230
    Keywords: polyenic oligomerics ; dipole moments ; one-dimensional molecular wire model ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Ground-state dipolar moments of oligomeric compounds, containing electron-donor (D) and electron-acceptor (A) groups as two terminal units of the polyenic bridge (D-wire-A), can well be described by means of a one-dimensional molecular wire model, which considers a scattering process of electrons through the charge-transfer conduction channel. The dipole moment of the oligomers (μn) follows a non-linear dependence of the polyenic bridge length (L) according to μn = μ0 + μ∞(1 - e- γL) where μ0 is the dipole moment of the first compound of the series, without a polyenic unit (n = 0), μ∞ is a limit value for L → ∞ and γ is the one-dimensional conduction constant of the π-molecular orbital channel of the molecular wire. This model can be extended to all those conjugated oligomers of the D-wire-A type where the electronic charge of the donor group can induce a soliton wave as far as through the polyenic bridge towards the acceptor group. Copyright © 1998 John Wiley & Sons, Ltd.
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    Conformational effects on the mechanism of acid-catalyzed dehydration of hexitols (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 887-894 
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    ISSN: 0894-3230
    Keywords: D-glucitol ; D-mannitol ; conformation ; dehydration reaction ; molecular mechanics ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The Monte Carlo search of the low-energy regions of D-mannitol (4) and D-glucitol (5) conformational space was undertaken in order to investigate conformational effects on the stereochemistry of the acid-catalyzed dehydrative cyclization reaction of 4 and 5 to the corresponding 1,4:3,6-dianhydrohexitols. It was found that although dominant conformations in water resemble closely the crystal state conformations of 4 and 5, the same is not true in solvents of low polarity. The magnitudes of the calculated energies of the reactive conformations for the dehydrative cyclization reaction suggest an SN2 mechanism with a late transition state. Copyright © 1998 John Wiley & Sons, Ltd.
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    Photochemical removal of the tosyl group from the 5′N position of 5′-aminopyrimidine nucleosides: syntheticapplications (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 618-621 
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    ISSN: 0894-3230
    Keywords: 5′-aminopyrimidines ; p-toluenesulfonyl photoremoval ; 3′-azido-3′-deoxythymidine derivatives ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The p-toluenesulfonyl (tosyl) group, an effective protector of the amine function of thymidine derivatives 2a and b, has proven to be photoremovable. This photoreaction was successfully used in the synthesis of new 5′-amino analogs of 3′-azido-3′-deoxythymidine (AZT), 6a and b. Selective photohydrolysis of 5′N-tosylamides 2a and b was carried out by UV irradiation (〉300 nm) in aqueous acetonitrile in the presence of 1,5-dimethoxynaphthalene as an electron donor. © 1998 John Wiley & Sons, Ltd.
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    Hydrolysis of N-methyl-N-nitroso-p-toluenesulphonamide in micellar media (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 584-588 
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    ISSN: 0894-3230
    Keywords: N-methyl-N-nitroso-p-toluenesulphonamide ; hydrolysis ; micellar media ; pseudophase model ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The acid and basic hydrolysis of N-methyl-N-nitroso-p-toluenesulphonamide (MNTS) was studied in different micellar aggregates (CTACl, CTABr, TTABr, LTABr, and OTACl). The effect of mixed micelles (OTAX-LTAX) was also studied. The kinetic behaviour was explained on the basis of the pseudophase model, from which the binding constants of MNTS to different micelles, the ion exchange constants and the reactivity constants in the micellar pseudophase were obtained. © 1998 John Wiley & Sons, Ltd.
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    Binding of protons and zinc ions to transition states for tautomerization of α-heterocyclic ketones: implications for enzymatic reactions (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 519-528 
    Details
    ISSN: 0894-3230
    Keywords: proton binding ; zinc ion binding ; transition states ; α-heterocyclic ketones ; tautomerization ; enzymatic reactions ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The description of catalysis in terms of binding of a catalyst to the transition state propoposed by Kurz is applied to tautomerization of the α-heterocyclic ketones phenacylpyridine, phenacylpyrazine, phenacylphenanthroline and phenylacetylpyridine catalysed by protons and zinc ions. Binding constants for protonated and zinc-coordinated transition states, KB≠ are reported and Brønsted coefficients are calculated from comparison of KB≠ with binding constants for the keto reactant and enolate anion intermediate. The formal equivalence of the binding formalism to a conventional Brønsted analysis is emphasized, and the results are compared with those from a ‘generalised’ Brønsted plot of rate constants against equilibrium constants for reactions of uncomplexed, protonated and zinc ion-coordinated ketones. This plot confirms that intrinsic reactivities of metal-coordinated and protonated substrates are similar even where differences exist between substrates. Application of a comparable Kurz-Brønsted treatment to enzymatic reactions depends in principle upon (a) dissecting binding contributions to catalysis from approximation of covalently reacting groups and (b) separating binding at the reaction site of the substrate, to which Kurz's treatment applies, from ‘remote’ binding, which, to a first approximation, is unchanged between Michaelis complex and transition state. The Brønsted relationship highlights stabilization of reactive intermediates as a thermodynamic driving force for binding catalysis at the reaction site. A formal expression which describes this stabilization, and also accommodates stabilization by remote binding of the substrate and intermediate by the enzyme, is proposed. Its relationship to the usual expression for application of the Kurz approach to enzyme catalysis, (kcat/k0)/Km = KB≠, is discussed and the usefulness of the Brønsted and Marcus relationships for interpreting KB≠ is emphasized. © 1998 John Wiley & Sons, Ltd.
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    Mono- and polynuclear ruthenium(II) complexes, photo-probes and reagents for targeted DNA sites (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 566-576 
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    ISSN: 0894-3230
    Keywords: ruthenium (II) complexes ; photoreactivity ; targeted DNA sites ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Several luminescent ruthenium(II) complexes were designed whose main characteristic is their photoreactivity towards mononucleotides and DNA. It was clearly demonstrated that this photoreactivity originates from a photoinduced electron transfer from a guanine to the excited complex. This process leads to the formation of an adduct which was characterized. The structure shows that the complex is anchored to the nucleotidic base via one of its polyazaaromatic ligands, thus marking irreversibly the DNA guanines. Interestingly, this property can be used in order to target, for example, (i) specific DNA sequences and (ii) particular DNA topologies. For each purpose a specific Ru(II) complex was designed. Synthetic oligonucleotides derivatized with mononuclear complexes were prepared to target and damage specific DNA sequences containing G sites. In these systems, it is shown that the DNA damage consists of an irreversible photo-crosslinking of the derivatized oligonucleotide with the complementary strand. In order to target portions of important deformation along double-stranded DNA, the dinuclear complex [Ru(phen)2]2HAT4+ was prepared and studied. This complex is too large to penetrate inside the major or minor grooves of a DNA double helix, so that only single-stranded portions of denatured DNA are accessible to this compound. © 1998 John Wiley & Sons, Ltd.
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    Linear free enthalpy relationships: a powerful tool for the design of organic and organometallic synthesisDedicated to Professor Reinhard W. Hoffmann on the occasion of his 65th birthday. Revised version of a Plenary Lecture presented at ESOR VI, Louvain la Neuve, July 1997. (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 642-654 
    Details
    ISSN: 0894-3230
    Keywords: linear free energy relationships ; organic synthesis ; organometallic synthesis ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Structure-reactivity relationships are usually investigated within relatively narrow classes of compounds, where structural variation can unequivocally be assigned to certain electronic and steric effects. We present a novel, semiquantitative approach to predict possible reactions of cationic electrophiles (carbenium ions, metal π-complexes, diazonium ions) with neutral nucleophiles (alkenes, alkynes, arenes, hydrides, organometallics and n-nucleophiles). It is shown that the rate constants for these reactions are given with a precision better than a factor of 10-100 by the linear free enthalpy relationship log k(20 °C) = s (E + N), where E characterizes the strengths of the electrophiles, N characterizes the strengths of the nucleophiles and s is a nucleophile-dependent slope parameter, usually close to 1. This deviation appears tolerable in view of the reactivity range extending over more than 30 orders of magnitude, the large structural variety of compounds included and the neglect of solvent and steric effects. The simultaneous treatment of aliphatic, aromatic and organometallic compounds, which becomes possible in this way, provides new qualitative insights, and it is shown how the rule of thumb that reactions proceed at 20 °C if E + N 〉 -5 can be used for rationalizing and designing organic reactions. © 1998 John Wiley & Sons, Ltd.
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    A perspective on biological catalysis (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 508-511 
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    ISSN: 0894-3230
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
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    Radical bromination of 1,1- and 1,2-diphenylethylenes in 1,2-dichloroethaneDedicated to the memory of Professor Giuseppe Bellucci (dièd 3 March 1996). (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 685-692 
    Details
    ISSN: 0894-3230
    Keywords: radical bromination ; diphenylethylenes ; 1,2-dichloroethane ; caged radical intermediates ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The radical bromination of 1,1- and 1,2-diphenylethylenes in 1,2-dichloroethane was investigated on the basis of kinetic and product distribution data. Whereas the ionic process followed a third-order rate law (second order in Br2), the radical bromination was second order in Br2 and zero order in olefin in the reagent concentration range examined. Significant inverse kinetic isotope effects were found for the bromination of 3,4′-bis(trifluoromethyl)-1,1-diphenylethylene and cis-1,2-diphenylethylene under these conditions. At variance with the ionic bromination, which gave mixtures of meso- and d,l-dibromides only the meso-dibromides were obtained both from the cis- and trans-1,2-diphenylethylenes. The kinetic and product distribution data for the radical process are discussed in terms of a mechanism in which caged radical intermediates are formed. © 1998 John Wiley & Sons, Ltd.
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    Kinetic mechanism for dimerization of an α-thioamide substituted benzyl carbocation in aqueous solution (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 701-706 
    Details
    ISSN: 0894-3230
    Keywords: carbocation ; thioamide ; kinetics ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The products of reaction of the α-(N,N-dimethylthiocarbamoyl)-4-methoxybenzyl carbocation (1+) intermediate of solvolysis of α-(N,N-dimethylthiocarbamoyl)-4-methoxybenzyl benzoate esters (1-O2CAr) show a strong dependence on solvent. The only product from reaction in 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) is 2-dimethylamino-6-methoxybenzothiophene (2) from intramolecular cyclization of 1+. The reaction of 1+ in 50:50 (v/v) methanol-water (I = 0.50, NaClO4) gives mainly the adducts to solvent. In 50:50 (v/v) trifluoroethanol-water (I = 0.50, NaClO4), 1+ partitions between reaction with solvent (ks, 27% yield), cyclization to form 2 (kc, 3% yield) and nucleophilic addition of 2 to 1+ (kalk, 70% yield) to form dimeric product 3. The yield of 3 in 50:50 (v/v) trifluoroethanol-water (I = 0.50, NaClO4) is independent of the leaving group at 1-O2CAr and remains constant as the concentration of the substrate is increased fourfold. These data show that the rate-determining step for dimerization of 1-O2CAr is ionization of substrate to form 1+ and that the products of the reaction are determined by the rate constant ratio for partitioning of 1+ between addition of solvent and cyclization. The rate constant ratios determined for the partitioning of 1+ in 50:50 (v/v) trifluoroethanol-water (I = 0.50, NaClO4) were ks/kc = 0.8 M-1 and kalk/ks = 540 000 M-1. © 1998 John Wiley & Sons, Ltd.
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    Substituent effects on the electrochemical oxidation of N,N′,N″-triphenyl-1,3,5-triaminobenzenes (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 731-736 
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    ISSN: 0894-3230
    Keywords: substituted 1,3,5-triaminobenzenes ; electrochemistry ; linear free energy correlation ; substituted triphenylamines ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Correlation analysis of the oxidation potentials of a series of N,N′,N″-triphenyl-1,3,5-triaminobenzenes (TPABs) substituted at the para positions of the outer phenyl rings shows a linear free energy relationship with resonance-enhanced substituent parameters (σ+). Reaction parameters (ρ+) for oxidation of TPABs were found to be -1.53, -1.45 and -1.34 (per substituent) in methylene chloride, acetonitrile and propylene carbonate respectively. The resonance enhancement and small magnitude of the ρ+ values are related to a significant but weak delocalization of charge onto the outer phenyl rings in the molecular orbitals of radical cations resulting from the oxidation of TPABs. Data on the oxidation of p-substituted triphenylamines were treated similarly and gave a ρ+ value of -3.27 (per substituent) in acetonitrile, greater than that for TPABs owing to a more significant delocalization of charge onto the phenyl rings in the molecular orbitals of the corresponding radical cations. To demonstrate their predictive value, these linear free energy correlations were used to estimate the oxidation potentials of similarly substituted N,N,N′,N′,N″,N″-hexaphenyl-1,3,5-triaminobenzenes, which are of interest as building blocks for molecular magnetic materials. © 1998 John Wiley & Sons, Ltd.
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    Electrophilic aromatic substitutions: reactions of hydroxy- and methoxy-substituted benzenes with 4,6-dinitrobenzofuroxan: kinetics and mechanism (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 707-714 
    Details
    ISSN: 0894-3230
    Keywords: electrophilic aromatic substitutions ; 4,6-dinitrobenzofuroxan ; σ-complexation ; superelectrophile ; nitrobenzofuroxan ; kinetics of SEAr reaction ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Rate constants have been determined in aqueous Me2SO mixtures for the reaction of super-electrophilic 4,6-dinitrobenzofuroxan (DNBF) with a series of hydroxy- and methoxy-substituted benzenes whose pKa values range between -3 and -9. The study extends the reactivity range of weakly basic aromatics with DNBF, from the family of indoles previously studied with pKa values ranging from -1 to -6. The overall rate constants for the reactions of DNBF as the electrophile are at least one order of magnitude greater than for the reactions of H3O+ with the same series of aromatics. This lends further credence to the notion that DNBF possesses super-electrophilic properties. An LFER is observed between logk50%Me2SODNBF and pKaH2O with slope 0.54. In the case of 1,3,5-trimethoxybenzene a significant kinetic isotope effect (KIE) is observed (kH/kD = 3.71 in 50% Me2SO). This system hence affords one of the few instances in which a KIE has been observed in SEAr reactions. It follows from the observed KIE that the addition of DNBF to the aromatic is not rate-limiting and that reversion to reactants and proton loss from the arenonium intermediate occur at comparable rates. Structures of the products of electrophilic substitution have been confirmed by 1H NMR. In all cases the regiochemistry of the reactions was identical to that observed in protonation studies of the starting aromatics. © 1998 John Wiley & Sons, Ltd.
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    Solvent dependence of pyranine fluorescence and UV-visible absorption spectra (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 743-750 
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    ISSN: 0894-3230
    Keywords: pyranine ; photoacid ; solvent polarity parameters ; solvatochromism ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Fluorescence and UV-visible absorption spectra of HPTS (8-hydroxy-1,3,6-pyrenetrisulphonic acid trisodium salt, pyranine) were measured in a variety of solvents. Fluorescence maxima (in kcal mol-1) can be correlated with the Kosower Z parameter (r = 0.901), the Dimroth-Reichardt ET(30) parameter (r = 0.900) and the Winstein Y parameter (r = 0.916) using one-parameter fits. Good correlations (r = 0.98) were obtained for HPTS fluorescence in ethanol-water mixtures using the Y, YOTs and Z parameters. Fluorescence maxima of HPTS in aqueous sulphuric acid solutions gave an excellent correlation with YOTs (r = 0.991). Multi-parameter correlations, indicating the significance of specific solvent interactions, were also studied. In addition, fluorescence maxima correlate well with maximum/minimum ratios obtained from UV-visible absorption experiments. Results can be applied in the use of HPTS as a molecular probe of solvent environments and for extension of the YOTs scale in acidic solutions. HPTS is a unique molecular probe, not only because of its photoacidic properties and its widespread use as a pH-sensitive biosensor, but also because of its relative stability in acidic environments. © 1998 John Wiley & Sons, Ltd.
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    Solvolysis-decomposition of N-1-adamantyl-N-p-tolylcarbamoyl chloride in hydroxylic solventsAbstracted, in part, from the MS thesis of Veena Upadhyay, Northern Illinois University, May 1993. (1997)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 600-606 
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    ISSN: 0894-3230
    Keywords: N-1-adamantyl-N-p-tolylcarbamoyl chloride ; solvolysis ; decompositions ; hydroxylic solvent ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: ---In hydroxylic solvents, N-1-adamantyl-N-p-tolylcarbamoyl chloride undergoes a facile rate-determing ionization with very little assistance from nucleophilic solvation (very low sensitivity to changes in solvent nucleophilicity). In the relatively nucleophilic aqueous ethanol or aqueous acetone, the cation formed reacts directly with the solvent. In fluoroalcohols, the low nucleophilicity of the solvent allows an effective competition from a pathway involving disproportionation to the 1-adamantyl cation and p-tolyl isocyanate, followed by reaction of the new cation either with solvent or with the chloride ion formed in the initial ionization. Grunwald-Winstein plots against YC1 values show for different binary solvent systems a marked dispersion, which can be considerably reduced by incorporation into the analyses of a term governed by the aromatic ring parameter (I). © 1997 John Wiley & Sons, Ltd.
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    Electron transfer properties of non-alternant, substituted compounds related to fluoranthene. Experimental determination and theoretical modeling of electrochemical oxidation and reduction potentials in non-aqueous solution (1997)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 623-630 
    Details
    ISSN: 0894-3230
    Keywords: Electron transfer properties ; substituted flouranthene derivatives ; redox potentials ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: ---The oxidation and reduction potentials of a series of related even non-alternant derivatives of 7,14-disubstituted acenaphth[1,2-k] fluoranthenes, and also fluoranthene, 7,10-diphenylfluoranthene and 8,9-dihydrodiindeno[1,2-j;2′,1′-] fluoranthene, were determined in organic solvents by cyclic voltammetry. The effects of steric hindrance on conjugation of the substituents with the central polycyclic aromatic hydrocarbon nucleus were evaluated. The semi-empirical molecular orbital calculation programs OMEGAMO, Extended Hückel, AM1 and PM3 were used to obtain optimal geometries and calculated HOMO and LUMO energies. As a further refinement, COSMO solvation was included in the AM1 calculations. The redox properties were correlated with data derived from the various semi-empirical calculations and the quality of these correlations is discussed. Inclusion of solvation energies in the computed molecular orbital energies results in a significant improvement in the correlation between observed and calculated oxidation potentials. © 1997 John Wiley & Sons, Ltd.
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    Molecular mechanics calculations (MM3) on silanes (1997)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 697-715 
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    ISSN: 0894-3230
    Keywords: silanes ; polysilanes ; molecular mechanics calculations ; MM3 ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: -Molecular structures, conformational energies, heats of formation and vibrational spectra have all been fit using the MM3 force field for a group of 48 silanes and polysilanes for which experimental data are available. © 1997 John Wiley & Sons, Ltd.
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    X-ray and vibrational studies of 8-aminoquinoline. Evidence for a three-center hydrogen bond (1997)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 680-686 
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    ISSN: 0894-3230
    Keywords: 8-aminoquinoline ; x-ray structure ; vibrational spectra ; three-center hydrogen bond ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: -The x-ray structure of 8-aminoquinoline indicates that one of the NH bonds of the NH2 group forms a three-center hydrogen bond, the major component being the intramolecular hydrogen bond and the minor component the intermolecular hydrogen bond. The IR and Raman spectra of 8-aminoquinoline and of its N-deuterated counterpart reflect the non-equivalence of the two NH(D) bonds and the weakness of the hydrogen bridge. © 1997 John Wiley & Sons, Ltd.
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    Chemical contrast in scanning near-field optical microscopy (1997)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 675-679 
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    ISSN: 0894-3230
    Keywords: chemical contrast ; scanning near-field optical microscopy ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: ---Repeatable and stable scanning near-field optical microscopy (SNOM) images are obtained under shear-force control if uncoated, i.e. cold and sharp, tapered fiber tips are used. True chemical contrast is seen for the first time on organic crystals due to the different near-field reflectivities of different chemical species on partly oxidized anthracene. The topography spans a Z-range of 500 nm. The surface roughness is tolerable and the sites of reaction can be related to crystal structure data. A submicroscopic local resolution of 18 nm has been obtained for chemical contrast, discriminating different chemical species even with poorly reflecting organics on rough crystal surfaces. Thus, reflection-back-to-the-fiber SNOM will find wide applications for both transparent and opaque samples. All drawbacks with blunt and hot metal-coated tips at a 〈10 nm distance from the surface in previous SNOM setups are overcome. © 1997 John Wiley & Sons, Ltd.
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    Electron transfer photochemistry of 7-(spirocyclopropane)quadricyclane: capture of a radical cation and sequential cyclopropylcarbinyl rearrangements (1997)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 607-611 
    Details
    ISSN: 0894-3230
    Keywords: electron transfer photochemistry ; 7-(spirocyclopropane)quadricyclane ; cyclopropylcarbinyl rearrangements ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: ---The electron transfer photo-sensitized reaction of 7-(spirocyclopropane)quadricyclane (1) with methanol produces two rearranged mono-methanol adducts, 2 and 3, and a bis-methanol adduct, 4. The products reveal that 1·+ reacts by stereo- and regiospecific attack of methanol on one trisubstituted cyclopropane ring. The resulting free radical rapidly undergoes one or two (consecutive) cyclopropylcarbinyl to butenyl rearrangements. The mono-adducts are formed by net hydrogen abstraction, the di-adduct via a (secondary) electron transfer reaction of 3. © 1997 John Wiley & Sons, Ltd.
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    Ab initio molecular orbital study of the conformational energies (-ΔG° or A values) of 4-alkyltetrahydro-2H-thiopyrans (tetrahydrothiopyrans, thiacyclohexanes, thianes) (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 831-839 
    Details
    ISSN: 0894-3230
    Keywords: conformational energy ; tetrahydrothiopyrans ; ab initio molecular orbital ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Ab initio 6-31G* and MP2/6-31G*//6-31G* methods were used to calculate the relative energies of the rotamers in the chair conformations of 4-alkyltetrahydro-2H-thiopyrans (tetrahydrothiopyrans, thiacyclohexanes, thianes; CH3, C2H5, i-C3H7, t-C4H9, neo-C5H11, SiMe3). The MP2/6-31G*//6-31G* conformational energies (-ΔG° or A values, kcal mol-1) of the 4-alkylthiacyclohexanes (Me = 1.94; Et = 1.72; i-Pr = 1.62; t-Bu = 5.49; neo-Pent = 1.39; SiMe3 = 2.76) are similar to those calculated for the corresponding alkylcyclohexanes. Plots of the calculated conformational energies for the 4-alkylthiacyclohexanes versus the calculated conformational energies for the corresponding alkylcyclohexanes are linear (slope = 1.01 and r = 0.999 for 6-31G* and slope = 1.09 and r = 0.990 for MP2/6-31G*//6-31G*). The conformational energies are strongly influenced by the magnitude and number of repulsive nonbonded hydrogen-hydrogen interactions and the gauche (synclinal) effects in the conformers. The carbon-sulfur bond lengths are mostly in the narrow range of 1.815-1.818 Å. The C - S - C angles vary from 97.1° to 99.4° and the C(3) - C(4) - C(7) angle in the most stable axial conformer is larger that the corresponding angle in its most stable equatorial conformer. © 1998 John Wiley & Sons, Ltd.
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    Non-cross-linked and cross-linked poly(alkylmethyldiallylammonium halides): synthesis and aggregation behavior (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 305-320 
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    ISSN: 0894-3230
    Keywords: poly(alkylmethyldiallylammonium halides) ; aggregation behavior ; polyelectrolytes ; polysoaps ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Hydrophobically modified polyelectrolytes (polysoaps) are a unique class of water-soluble polymers containing distinct hydrophobic and hydrophilic regions. Above a certain concentration, polysoaps form intramolecular and intermolecular aggregates in aqueous solution. They have attracted much attention not only for their ability to mimic some functions demonstrated by biopolymers but also for their important industrial applications. This review highlights some interesting features of novel non-cross-linked and cross-linked poly(alkylmethyldiallylammonium halides) that have been described in recent years. © 1998 John Wiley & Sons, Ltd.
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    Is the brain ready for physical organic chemistry? (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 334-340 
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    ISSN: 0894-3230
    Keywords: brain chemistry ; membrane proteins ; molecular neurobiology ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Efforts to developed detailed insights into the structure and function of the molecules of memory, thought and sensory perception - physical organic chemistry on the brain - are described. By combining more conventional chemical tools with a number of techniques adapted from modern biology, it is now possible to perform systematic structure-function studies on the integral membrane proteins that play a central role in molecular neurobiology. There are substantial challenges associated with such studies, but we believe the potential payoff is considerable. © 1998 John Wiley & Sons, Ltd.
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    Vicarious nucleophilic substitution of hydrogen. Mechanism and orientation (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 341-349 
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    ISSN: 0894-3230
    Keywords: vicarious nucleophilic substitution ; hydrogen ; mechanism ; orientation ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Hydrogens located at activated positions in electrophilic arenes, e.g. ortho and para hydrogens in nitrobenzenes, can be replaced with a nucleophile moiety provided there is at least one nucleofuge X connected to the nucleophilic centre. As the group really leaving in this hydrogen substitution process is not the hydride anion but X, the reaction has been named vicarious nucleophilic substitution of hydrogen (VNS). The concepts on the mechanism of the reaction and their experimental background are presented. Reactivity and orientation - the fundamental questions concerning synthetical applications of VNS - are discussed in light of the supposed mechanistic picture.
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    Preface (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 291-291 
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    ISSN: 0894-3230
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: No Abstract
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    New DNA-binding ruthenium(II) complexes as photo-reagents for mononucleotides and DNA (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 577-583 
    Details
    ISSN: 0894-3230
    Keywords: DNA binding agents ; ruthenium(II) complexes ; photoreagents ; DNA ; mononucleotides ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The spectroscopic properties of two photoprobes for DNA, Ru(phen)2(PHEHAT)2+ and Ru(TAP)2(PHEHAT)2+ (phen = 1,10-phenanthroline, TAP = 1,4,5,8-tetraazaphenanthrene, PHEHAT = 1,10-phenanthrolino[5,6-b]-1,4,5,8,9,12-hexaazatriphenylene), were examined and compared with those of complexes containing either an extended planar ligand (DPPZ) or π-acceptor ligands. The orbitals involved in the absorption and emission processes for Ru(phen)2(PHEHAT)2+ imply the PHEHAT ligand, whereas the chromophore and luminophore for Ru(TAP)2(PHEHAT)2+ are associated with the Ru(II) → TAP MLCT transition. The two complexes exhibit completely different behaviour in the presence of DNA. Whereas Ru(phen)2(PHEHAT)2+, which does not emit in water, luminesces upon intercalation between the DNA base pairs, the luminescence of Ru(TAP)2(PHEHAT)2+ is quenched by binding to DNA. Emission quenching is also observed in the presence of GMP, with a quenching rate constant of 1.25 × 109 l mol-1 s-1. This strongly suggests the presence of a photo-induced electron transfer from the guanine residues of GMP or DNA to the excited complex and leads to the conclusion that this complex is a good DNA photoreagent. © 1998 John Wiley & Sons, Ltd.
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    Conjugate addition of water to α-carbonylcarbenes (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 610-613 
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    ISSN: 0894-3230
    Keywords: Conjugate addition ; α-carbonylcarbenes ; water ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: α-Carbonylcarbenes (2a-c) generated by UV photolysis of 2-diazophenylacetic acid (1a), its methyl ester (1b) and 4-diazo-3-isochromanone (1c) in aqueous solution undergo conjugate addition of water across the entire carbonylcarbene moiety to give enols (3a-c) of the corresponding α-hydroxycarbonyl compounds. These carbenes are long-lived, with microsecond lifetimes in aqueous solution. © 1998 John Wiley & Sons, Ltd.
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    Flash photolytic generation and investigation of short-lived reaction intermediates: a case study (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 292-298 
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    ISSN: 0894-3230
    Keywords: flash-photolysis ; short-lived reaction intermediates ; mandelic acid ; keto-enol tautomers ; enzyme-catalyzed racemization ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The advantage of adding more structure-diagnostic information to the simple detection of flash photolytically generated transient species by changes in UV-visible light absorbance is illustrated by a case study involving the mandelic acid keto-enol system. An early report based on preliminary evidence proposed that flash photolysis of phenyldiazoacetic acid produces the enol of mandelic acid by hydration of phenylhydroxyketene, itself generated by a photo-Wolff reaction of the diazo acid. Further examination, however, shows that this is only a minor route, and that the major pathway is a new enol-forming reaction involving what appears to be hydration of a carboxycarbene formed by dediazotization of the diazo compound. Hydration of phenylhydroxyketene is nevertheless the reaction by which mandelic acid enol is generated when esters of benzoylformic acid are the flash photolysis substrates. These mechanisms, and also identification of the enol as a tranisent species, are supported by detailed arguments involving acid-base catalysis, solvent isotope effects, and the use of oxygen-18 as a tracer. The work produces a keto-enol equilibrium constant for the mandelic acid system, pKE  =  16.19, and also acidity constants of the enol ionizing as an oxygen acid, pKEa  =  6.39, and the keto isomer ionizing as a carbon acid, pKKa  =  22.57. The bearing of these results on the enzyme-catalyzed racemization of mandelic acid is discussed. © 1998 John Wiley & Sons, Ltd.
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    Gas-phase thermolysis of N-methyl-N-phenyl-tert-butylsulfenamide and morpholinyl-tert-butylsulfenamide (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 407-410 
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    ISSN: 0894-3230
    Keywords: gas-phase thermolysis ; N-methyl-N-phenyl-tert-butylsulfenamide ; morpholinyl-tert-butylsulfenamide ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: N-Methyl-N-phenyl-tert-butylsulfenamide (MPSA) and morpholinyl-tert-butylsulfenamide (MOSA) were thermolyzed in a stirred-flow reactor at temperatures of 340-390 °C and pressures of 7-13 Torr, using toluene as carrier gas, at residence times of 0.3-1.3 s. Isobutene was formed in 99% yield through first-order reactions having the following Arrhenius parameters (A,s-1, Ea, kJ mol-1): MPSA, log A  =  12.41 ± 0.02, Ea  =  158.8 ± 0.2; MOSA, log A  =  12.91 ± 0.22, Ea  =  159 ± 3. It is proposed that the elimination of isobutene takes place by unimolecular reaction mechanisms involving polar, four-center cyclic transition states, forming S-unsubstituted thiohydroxylamines as co-products. Thermochemical parameters, estimated by semiempirical AM1 calculations, are reported for the latter and for the parent molecules. © 1998 John Wiley & Sons, Ltd.
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  • 96
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    Electronic Resource
    NMR and semi-empirical study of the tautomerism of­2,2′-bisbenzimidazolyl (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 411-418 
    Details
    ISSN: 0894-3230
    Keywords: 2,2′-bisbenzimidazolyl ; tautomerism ; NMR ; semi-empirical calculations ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: A dynamic NMR study of the tautomerism of 2,2′-bisbenzimidazolyl in DMSO-d6 and a mechanistic interpretation of the process, based on a stepwise, single-proton transfer and formation of a zwitterionic intermediate, are presented. This interpretation is substantiated by semi-empirical calculations of the postulated intermediate and transition state, that yield results which are compared with previous studies on related aliphatic systems. © 1998 John Wiley & Sons, Ltd.
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  • 97
    Electronic Resource
    Electronic Resource
    Effects of sulfenyl, sulfinyl and sulfonyl groups on acidities and homolytic bond dissociation energies of adjacent C - H and N - H bonds (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 397-406 
    Details
    ISSN: 0894-3230
    Keywords: bond dissociation energy ; acidity ; phenylsulfenylamides ; phenylsulfenylanilides ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Acidities and bond dissociation energies (BDEs) of the N-H bond in two phenylsulfenylamides, PhSNHBz and PhSNH-t-Bu, and four phenylsulfenylanilides, 4-GC6H4NHSPh, where G  =  MeO, H, Br and CN, were measured in order to compare the effects of substituents on acidities and BDEs of N - H bonds with those of C - H bonds. The effects of PhS groups on acidities and BDEs in a series of C - H acids were found to be comparable to those on acidities and BDEs of PhS in a similar series of N - H acids. Comparisons were also made of the effects of changing the oxidation state of sulfur in the series PhS, PhSO and PhSO2 on the acidities and BDEs of adjacent N - H and C - H bonds in weak acids. Hammett-type plots of pKHA values for phenyl benzyl sulfones (4-GC6H4CH2SO2Ph) and phenylsulfenylanilides (4-GC6H4NHSPh) were linear vs σp- values. A linear plot was obtained and explained for a plot of BDE of the N - H bonds in remotely substituted phenylsulfonylanilides with σ+ values. Plots of BDEs vs Eox(A-) were also linear for 4-substituted phenylsulfenylanilides (4-GC6H4NHSPh), phenylsulfonylanilides (4-GC6H4NHSO2Ph) and phenyl benzyl sulfones (4-GC6H4CH2SO2Ph). © 1998 John Wiley & Sons, Ltd.
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  • 98
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    Electronic Resource
    17O NMR studies on α-diamides (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 387-391 
    Details
    ISSN: 0894-3230
    Keywords: α-diamides ; 17O NMR ; conformation ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: 17O NMR spectra of several α-diamides were obtained at natural isotopic abundance in acetonitrile solution in order to study the conformations of these compounds in solution. The 17O NMR shifts vary with the intercarbonyl dihedral angle α in a sense opposite to that observed for α-diketones. © 1998 John Wiley & Sons, Ltd.
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  • 99
    Electronic Resource
    Electronic Resource
    Theoretical study of the tautomerism of 8-azaadenine (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 392-396 
    Details
    ISSN: 0894-3230
    Keywords: 8-azaadenine ; tautomerism ; theoretical study ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The prototropic tautomerism of 8-azaadenine (azaade) was studied theoretically by means of ab initio methods, in both the gas phase and aqueous solution. A number of tautomeric forms were not included in the calculations after applying a stepwise elimination procedure based on both AM1 and HF/6-31G* energy values. The tautomers 9H-azaade, 8H-azaade and 7H-azaade survived to this elimination and their optimized geometries and energies were calculated at the MP2/6-31*//HF/6-31G* level. To include the solvent effects, two self-consistent reaction field method were used: (1) Onsager's SCRF with multipolar expansion up to the hexadecapolar term and (2) the isodensity polarizable continuum method (IPCM). Both methods produce similar results, although the latter represents better the situation in aqueous solution. The stability order in solution, 8H- 〉 9H- 〉 7H-azaade, differs slightly from that found in the gas phase, implying that in general the electrostatic effects in solution are important, but the intrinsic stability of these species in the gas phase overcomes the solvent effect. © 1998 John Wiley & Sons, Ltd.
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  • 100
    Electronic Resource
    Electronic Resource
    Host-guest interactions of calix[4]resorcinarenes with benzene derivatives in conditions of reversed-phase high-performance liquid chromatography. Determination of stability constants (1998)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 426-437 
    Details
    ISSN: 0894-3230
    Keywords: calix[4]resorcinarenes ; organophosphorus compounds ; solvatophobic interactions ; host-guest complexation ; high-performance liquid chromatography ; x-ray analysis ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Reversed-phase high-performance liquid chromatography [LiChrosorb RP-18,UV detection at 254 nm and acetonitrile-water (86:14,v/v) as mobile phase] was applied to studies of the host-guest complexation of tetraalkylcalix[4]resorcinareneoctols and their upper rim phosphoryl, sulfonyl and dialkylaminomethyl derivatives with some aromatic guests in the mobile phase. It was shown that the formation of the inclusion complexes results in changes in the retention of aromatic guests and improves their separation. Stability constants of the complexes were calculated from the dependences of the l/k′ values of the aromatic guest on the concentration of the calix[4]resorcinarene in the mobile phase. The molecular structure of 4,6,16,18-tetrahydroxy-10,12,22,24-tetrakis(dipropoxyphosphoryloxy)-2,8,14,20-tetramethylcalix[4]resorcinarene (12) was determined. Crystal data for 12 are P21/n, a  =  16.708(9) Å, b  =  18.683(6) Å, c  =  20.243(5) Å, β  =  95.75(3)°, V  =  6287(4) Å3 and Z  =  4. Compound 12 exists in a boat conformation, in which two opposite unsubstituted resorcinol rings of the macrocyclic skeleton lie in the plane formed by four methine bridges and two diphosphorylated rings are perpendicular to the plane. © 1998 John Wiley & Sons, Ltd.
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