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  • ab initio calculations  (44)
  • Wiley-Blackwell  (44)
  • 1
    ISSN: 0947-6539
    Keywords: ab initio calculations ; azaenolates ; NMR chemical shifts ; peptides ; Ramachandran maps ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Structures and conformations of the azaenolate lithium salts of amides (formamide, acetamide, and N-methylacetamide) and of the dipeptide model N-formylalaninamide were investigated by means of ab initio MO theory. Four possible structures of the lithiated C-enolates of acetamide were also included in the study. All structures were calculated at the HF/6-31+G(d) and MP2(fc)/6-31 + G(d)/HF/6-31 + G(d) levels; the lithiated azaenolates of formamide were also investigated at higher theoretical levels (up to MP4(fc)/6-311 + G(d,p)/MP2(fc)/6-311 + G(d,p)). For the lithiated azaenolates of all amides investigated, the most stable structure contains a four-membered ring in which the lithium ion is complexed by the oxygen and nitrogen atoms; the substituents attached to the carbon and nitrogen atoms of the azaenolate are in a cis arrangement. The lithiated azaenolates of acetamide are predicted to be more stable than the corresponding C-enolates. To simulate solvation, calculations on complexes of the lithiated azaenolates of formamide with up to three molecules dimethyl ether were also performed, and all azaenolates of amides were also reoptimized by ab initio reaction-field calculations. Both solvation models reduce the preference for lithium-chelated cis structures. The Ramachandran maps of the dilithiated bis(azaenolate) of N-formylalaninamide (having cis or trans arrangements of the azaenolate substituents) were scanned by MNDO calculations for conformational accessible regions. Thirteen stable structures were subsequently optimized at the HF/6-31 + G(d) ab initio level. The global minimum resembles a peptide in C7 conformation, but other conformations, not known for peptides, are close in energy. The structures of dimers of the lithiated azaenolates of N-methylacetamide and of glycinaldehyde were also calculated. The NMR chemical shielding of carbon, nitrogen, and oxygen atoms in all structures were predicted ab initio by using the gauge-including atomic orbital (GIAO) method.
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  • 2
    ISSN: 0947-6539
    Keywords: ab initio calculations ; cyclic ylides ; heterocycles ; phosphorus ylides ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A number of possible strategies for the preparation of λ5-phosphetes were tested as follows: cyclopropenium 6 was treated with the lithium salt of diphenylphosphine to give phosphinocyclopropene 7, but 7 did not undergo ring expansion upon photolysis or thermolysis. P-chloro-C—trimethylsilyl-substituted ylide 8b reacted with two equivalents of dimethyl acetylenedicarboxylate to afford phosphinine 13 via a transient λ5-phosphete 12. Addition of aluminum trichloride to P-halogenated ylides 17a-b led to dihydrophosphetium salts 19a-b, which, upon treatment with pyridine, isomerized into the 1, 2-dihydrophosphet-2-ium salts 20a-b. Hydrolysis of derivatives 20a-b cleanly afforded phosphoniums 21 a-b, which reacted with NaN(SiMe3)2 to give rise to the corresponding λ5-phosphetes 22a-b. The benzo-λ5-phosphete 22a underwent ring expansion reactions with dimethyl acetylenedicarboxylate and acetonitrile, leading to benzo-λ5-phosphinine and benzo-1, 4λ5-azaphosphinine in good yields. Derivative 22b was characterized by X-ray crystal structure analysis. Ab initio SCF calculations, IGLO-13C chemical shifts and Δχ for various benzannulated derivatives and phosphorus heterocycles are presented.
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  • 3
    ISSN: 0947-6539
    Keywords: ab initio calculations ; conformation ; microwave spectroscopy ; pyridines ; stereoelectronic effect ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: One single conformer was assigned from the microwave spectrum of 2-(fluoromethyl)pyridine, investigated in the gas phase in the 26.0-39.0 GHz spectral region at about -10°C. Its Cα-F bond was found to be coplanar with the ring and anti to the N-C2 bond (syn to the C2-C3 bond). There was no indication in the microwave spectrum of the presence of other rotameric forms of the molecule.The results of the spectroscopic study were backed up by ab initio calculations at the MP2/6-31 G** (frozen core) level. These calculations predict that the assigned conformer is the only stable form of the molecule. The transition state was calculated to have the CH2F group 180° from the stable anti conformation. The energy of the transition state was computed to be 20.2 kJ mol-1 higher than the energy of the anti rotamer.The results are interpreted in terms of a stereoelectronic effect, and the orbital overlaps responsible for the observed effect are discussed. It is shown that 2-(fluoromethyl)pyridine serves as a good model for 2-(alkoxymethyl)pyridines, previously found to show the same conformational preference.
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  • 4
    ISSN: 0947-6539
    Keywords: ab initio calculations ; conformation ; sigma conjugation ; matrix isolation ; oligosilanes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Infrared and ultraviolet spectra of the gauche and anti conformers of matrix-isolated permethyl-n-tetrasilane have been obtained separately by taking advantage of thermally induced gauche-to-anti conversion and of wavelength-selective photochemical destruction of either conformer. The resolved UV spectrum of the gauche conformer provides the first piece of experimental evidence in favor of the recently proposed reinterpretation of conformational effects on tetrasilane electronic states. According to this, it is not the energy but the intensity of the lowest singlet excitation that changes dramatically as the SiSiSiSi dihedral angle is varied, as a result of an avoided crossing between s̰s̰* and s̰π* states. Implications for the general understanding of sigma conjugation in simple terms are discussed. Unconstrained MP2/6-31 G* optimization predicts the existence of a third backbone conformer (ortho), with a dihedral angle of about 90°. Its predicted (HF/3-21 G*) mid-IR spectrum is indistinguishable from that of the gauche conformer, and the matrix-isolation spectra thus provide no evidence for or against its presence.
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  • 5
    ISSN: 0947-6539
    Keywords: ab initio calculations ; Henry reaction ; nitroaldol reactions ; reaction mechanisms ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Ab initio calculations (up to MP4SDQ/6-31+G*//MP2(FU)/6-31+G* + δZPVE) on several model nitroaldol (Henry) reactions have been performed. It is found that the free nitronate anions react with aldehydes via transition states in which the nitro and carbonyl dipoles are antiperiplanar to each other. This kind of reaction yields anti (erythro) nitroalcohols as major products. The Henry reaction between lithium nitronates and aldehydes is predicted to occur via cyclic transition structures yielding syn nitroalcohols as major products. The stereocontrol in these model reactions is low. The factors affecting the stereoselectivity in the reaction between dilithiated nitronates and aldehydes are also discussed.
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  • 6
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 237-248 
    ISSN: 0947-6539
    Keywords: ab initio calculations ; allenes ; ketenes ; matrix isolation ; rearrangements ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Alkoxyvinylketenes4are generated by flash vacuum thermolysis (FVT) or photolysis of 3-alkoxycyclobutenones3. The thermal interconversion of4and allene carboxylic acid esters5under FVT conditions is demonstrated by Ar matrix FTIR spectroscopy. In addition, ethoxy-vinylketene4bundergoes thermal elimination of ethene with formation ofs-cis-ands-trans-acetylketene(8). An analogous aminovinylketene-to-allenecarbox-amide conversion is observed on FVT of 3-dimethylaminocyclobutenone3c. A facile 1, 3-chlorine migration in 2, 3-buta-dienoyl chloride(5d)is also reported. Consistent with the experimental observations, 1, 3-methoxy, 1, 3-chloro, and 1, 3-dimethylamino migrations in vinylketene are calculated (G2(MP2, SVP) level) to have moderate barriers of 169, 157, and 129 kJ mol-1, respectively, significantly less than the corresponding 1, 3-H shift barrier (273 kJ mol-1). The stabilization of the four-center transition structures is rationalized in terms of the donor-acceptor interaction between the lone pair electrons of the migrating donor substituent and the vacant central carbon p orbital of the ketene LUMO. The predicted migratory aptitude in the series of substituted vinylketenes, R-C(=CH2)-CH=C=O, is in the order N(CH3)2〉SCH3〉SH〉Cl〉NH2〉OCH3〉OH〉F〉H〉CH3, and correlates well with the electron-donating ability of the R group.
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  • 7
    ISSN: 0947-6539
    Keywords: ab initio calculations ; cyclizations ; cycloketones ; radicals ; ring expansions ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Alk-5-enoyl radicals were made to cyclize in exo and endo modes to give the corresponding cycloketone radicals, which are related through one-carbon ring expansion. Relative kinetic data were determined for the ring closure of the 2-methylhept-5-enoyl radical generated by the reaction of the corresponding phenylseleno ester with Bu3SnH over the temperature range 233-323 K. The conversion to absolute rates provided Arrhenius expressions for the 5-exo-trig and 6-endotrig cyclizations. Ab initio and semiempirical (AM1) calculations were performed on the hex-5-enoyl and hept-5-enoyl radicals, respectively, and the outcomes aided in the rationalization of the preexponential factors and activation energies. Both 1,5-and 1,6-ring closure occur via a lower energy “chairlike” transition state. The observed high regioselectivity is due to favorable entropic and enthalpic factors associated with the formation of the smaller ring. The stereoselectivity was higher in the 1,6-ring closure (70:30) than in the 1,5-ring closure (55:45), the trans isomer being predominant in both. For the onecarbon ring expansion studies, the radicals of interest were obtained by deoxygenation of suitable alcohols via the O-phenyl thiocarbonates with (TMS)3-SiH. The one-carbon ring expansion in the cyclopentanone series for the secondary alkyl radicals was studied over the temperature range 343-413 K by means of free-radical clock methodology and yielded the Arrhenius expression. The rate constant was 4.2 x 103 s-1 at room temperature and the reverse reaction (ring contraction) was found to be at least 10 times slower. Since the intermediacy of acyl radicals can be excluded, the reaction must occur via 3-membered cyclic intermediate radicals (or transition states).
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  • 8
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 717-725 
    ISSN: 0947-6539
    Keywords: ab initio calculations ; chromophores ; dyes ; electronic structure ; indigo ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A theoretical study of the electronic spectra of indigo, bispyrroleindigo, and the H-chromophore, thought to be the basic structure leading to the intense absorption in the visible spectrum of the indigoid dyes, has been performed by means of the complete active space (CAS) SCF method and the multiconfigurational second-order perturbation theory (CASPT2). The calculations include excitation energies, oscillator strengths, and transition moment directions for the valence singlet states of the molecules. The assumption that the H-chromophore is responsible for the absorption in the visible spectrum is quantitatively revisited with the help of the modern ab initio methodologies and the main features of these molecules are theoretically analyzed, some of them for the first time.
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  • 9
    ISSN: 0947-6539
    Keywords: ab initio calculations ; asymmetric alkylations ; catalysis ; structure ; elucidation ; zinc complexes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Dimethylzinc reacts with (2S)-or (2R)-3-exo-(dimethylamino)isoborneol [(2S)- or (2R)-DAIB] to eliminate methane and produce a tricoordinate methylzinc aminoalkoxide, which forms a dimeric structure. The homochiral dimerization of the enantiomeric compound leads to the chiral, (S,S) or (R,R) dinuclear Zn complex, while the heterochiral interaction forms the meso (S,R) dinuclear compound. In both solution and crystalline state, the heterochiral dimer is more stable than the homochiral dimer. This stability difference in solution is the origin of the chirality amplification observed in the amino alcohol promoted asymmetric addition of dimethylzinc to benzaldehyde. In toluene, the homochiral dimer dissociates more readily into the monomer than the heterchiral isomer and also undergoes dissociation of the N-Zn dative bond making the two N-methyl groups equivalent. The differences in solution behavior between the diastereomers can be understood by comparing their crystal structures. X-ray analysis indicates that the labile Zn-O and Zn-N bonds in the (S,S) dimer are longer than those in the (S,R) isomer. Skeletal congestion caused by the polycyclic framework is the prime factor determining the properties of the dinuclear Zn complexes, with both steric and electronic factors governing their geometries. The distances between the C-2 proton and N-CH3 of the other DAIB moiety in the homochiral dimer are close to the sum of the van der Waals radii. A significant nuclear Overhauser effect is seen between these protons in the homochiral dimer. The tetrahedral Zn atoms in the dinuclear complexes are linked covalently to the methyl group, to two oxygen atoms through covalent/electrostatic hybrid bonds, and to the dimethylamino group through electrostatic interaction. The repulsive interaction of the 1, 3-synoriented Zn-CH3 bonds significantly contributes to the lower stability of the homochiral dimeric complex. The N-Zn interaction in the homochiral dimer is labile, owing to the increase in the electrostatic interaction between the Zn atom and the neighboring oxygen atoms. This view is supported by the ab initio molecular orbital calculations of the model systems.
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  • 10
    ISSN: 0947-6539
    Keywords: ab initio calculations ; arsenic ; silicon ; phosphorus ; photoelectron spectroscopy ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Dodecamethyl-2,3,5,6,7,8-hexa-sila-lλ3,4λ3-diphosphabicyclo[2.2.2]oc-tane (1) and its arsenic analogue 2 are readily accessible in 69 and 73% yield, respectively, by the cyclocondensation reaction of 1,2-dichloro-1,1,2,2-tetrame-thyldisilane (5) with the lithium pnictides [LiEH2(dme)] (E = P (6), As(7); dme = 1,2-dimethoxyethane). The reactions proceed via 1,4-diphosphaoctamethyltetrasi-lacyclohexane (8) and its arsenic analogue 9, respectively, which were isolated and structurally characterized by X-ray crystallography. The molecular structures of 1 and 2, which are isotypic, were also established by single-crystal X-ray analysis: they possess D3 point symmetry with the expected Si-E bond lengths (E = P, As) but unusually long Si-Si bonds. The latter are 0.02-0.03 Å longer than those in 8 and 9, mainly due to through-bond interactions (TB) between donating n orbitals of the E atoms and the σ* acceptor orbitals of the Si-Si bond. The first expanded analogues of 1, namely, 12 and 14, with hexamethyltrisilane and dodecamethyl-hexasilane chains bridging the two phosphorus atoms, were synthesized in a onepot cyclocondensation reaction of the corresponding 1,3- and 1,6-dichloro-oligosilanes 11 and 13, respectively, with [LiPH2(dme)]6. Ab initio calculations on the parent compounds 1a, 12a, and the second-row analogue 1,4-diazabicyclo-[2.2.2]octane (B) were carried out in order to analyze the different coupling constants and magnitudes of intramolecular interactions (through-space/through-bond coupling). TS and TB coupling in B were found to be about two times stronger than in the congener 1a, due to the compactness of the N2C6 skeleton and the greater extent of s, p hybridization at nitrogen. Evidence for TB interactions in 1 was obtained by photoelectron spectroscopy and by comparison of the two first vertical ionization potentials with calculated values for 1a. The best agreement with experimental data was achieved when 1a was calculated at the MP2 level. Compound 1a preferentially adopts D3 point symmetry; the higher-symmetry D3h form possesses one imaginary frequency and is slightly less stable (0.46 kcal mol-1 at HF/6-31 G*//HF/6-31 G* and 1.58 kcal mol-1 at MP2/ 6-31 G*//HF/6-31 G* level), suggesting that this structure corresponds to a transition state on the potential energy surface. The structures corresponding to the global minimum of B and 12a have D3h and C3h symmetry, respectively. At the HF/6-31 G*//HF/6-31 G* level the D3h form of 12a is 17.61 kcal mol-1 less stable than the C3h minimum.
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