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  • ab initio calculations  (44)
  • self-assembly  (23)
  • clusters  (12)
  • 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: catenanes ; polycatenanes ; polyrotaxanes ; rotaxanes ; self-assembly ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The self-assembly of three bis[2]catenanes and a bis[2]rotaxane, by two complementary strategies, is reported. A synthetic route to derivatives of bis-para-phenylene[34]crown-10 (BPP34C10) and 1,5-naphtho-para-phenylene[36]-crown-10 (1/5NPP36C10) containing a fused five-membered ring with a secondary amine function is described. These intermediate N-allylimido macrocyclic polyethers undergo template-directed reactions with 1,1′-[1,4-phenylenebis-(methylene)]bis-4,4′-bipyridinium bis-(hexafluorophosphate) and 1,4-bis(bromo-methyl)benzene to produce [2]catenanes containing an N-allyl functionality. The N-allylimido macrocyclic polyethers have also been reduced and deprotected to afford macrocycles possessing a free NH group, which are then linked through a 4,4′-biphenyldicarbonyl spacer to produce bis(crown ether)s, in which each crown ether moiety has two recognition sites. These ditopic BPP34C10 and 1/5NPP36C10 derivatives are capable of sustaining self-assembly reactions at both recognition sites to yield bis[2]catenanes. The self-assembly of a complementary bis[2]catenane, in which two tetracationic cyclophanes are linked together with a flexible hexyl chain, has also been achieved by treating 1,1′-[1,4-phenylenebis(methylene)]bis-4,4′-bipyridinium bis-(hexafluorophosphate) with a compound containing two linked 1,4-bis(bromomethyl)benzene units in the presence of BPP34C10. Replacing BPP34C10 with a dumbbell-shaped compound containing a linear polyether unit intercepted by a naphthalene residue and terminated by two bulky adamantoyl groups has led to the self-assembly of a bis[2]rotaxane. The X-ray crystal structures of one of the catenanes and its associated crown ether component are reported, together with solution state dynamic 1H NMR spectroscopic studies, showing that there is substantial degree of order characterizing the molecular structure of the catenanes.
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  • 3
    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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  • 4
    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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  • 5
    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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  • 6
    ISSN: 0947-6539
    Keywords: clusters ; gas-phase chemistry ; ionmolecule reactions ; iron complexes ; mass spectrometry ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We report on the gas phase reactions of small Fe+n clusters (n = 2-10) with O2, N2O and CO2 in an FT-ICR mass spectrometer. Under our experimental conditions, clusters of all sizes reacted readily with O2 and all but the dimer reacted with N2O. Only the smallest Fe+n clusters (n = 2-4) appeared to activate CO2. For all X-O molecules (X = O, N2, CO), reaction pathways were observed that include the transfer of O atoms. In addition, the reactions with O2 and N2O were accompanied by the loss of one or two Fe atoms. Thermochemical considerations based upon the well-known X-O bond energies were used to calculate Fen-O+ bond dissociation energies (BDEs) for sizes n = 2-6; these amount to roughly 550 kJ mol-1 and thus are considerably higher than the BDE of the Fe-O+ ion. All oxidation reactions of the Fe+n clusters (n = 2-6) studied in more detail were terminated by products of FexOx+ stoichiometry (x = 1-4). These “microoxides of iron” are not able to activate any further X-O bonds. Secondary reactions of FexOx+ clusters with C6H6, C2H4 and NH3 were investigated for two selected sizes (x = 2, 3) and compared with reactions of the naked Fe+n clusters.
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  • 7
    ISSN: 0947-6539
    Keywords: lactam ; macrocycles ; rotaxanes ; self-assembly ; template syntheses ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Two new rotaxane types, the [2]rotaxanes 12a,b and the [3]rotaxane 13, have been synthesised by treating the “axle” 7 with the stopper component 9 in the presence of the “wheel” 8. A nonionic template intermediate of type 10 is proposed. The [3]rotaxane 13 was only obtained when the chain of the “axle” had a certain length: with n = 1 only [2]rotaxane 12 a was isolated, whereas with n = 2 the [3]rotaxane 13 was formed besides the [2]rotaxane 12 b. This suggests that more extended rotaxanes and polyrotaxanes can be synthesised by the template strategy.
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  • 8
    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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  • 9
    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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  • 10
    ISSN: 0947-6539
    Keywords: chromium ; clusters ; iron ; molybdenum ; tungsten ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A mixture of Na2MO4, M(CO)6, FeCl3·6H2O, and (EtCO)2O was heated at 120°C to produce the heterooctanuclear clusters Na2Fe2[M3O4-(O2CEt)8]2 (M = Mo, 1; W, 2). The bioxo-capped clusters Na[M3O2(O2CEt)9] (M3 = MoW2, W3) were treated with Fe-(CO)5, Cr(CO)6, and Mo(CO)6 in (EtCO)2O at 120°C to afford Na2M'2-[MoW2O4(O2CEt)8]2 (M' = Fe, 3; Cr, 4; Mo, 5) and Na2Mo2[W3O4(O2CEt)8]2 (6), respectively. The isomorphous clusters 1, 2, 5, and 6 were characterized by X-ray crystallography. The structures, similar to those of the analogues Na2Cr2[M3O4-(O2CEt)8] (M = Mo, 7; W, 8) and Na2V2[M3O4(O2CEt)8]2 (M = Mo, 9; W, 10), consist of two incomplete cubane-type [M3O4(O2Et)8]4- units centrosymmetrically bridged by two M'(III) metal ions through μ-oxo and propionate groups; the resulting cluster dianions are linked by Na+ ions into infinite chains. Clusters 1-10 do not dissolve in water and common organic solvents at room temperature. Unlike the chromium(III) clusters 7 and 8, the iron(III) and vanadium(III) clusters 1 and 9 are soluble in aqueous acid solution, as a result of their decomposition into isolated [Mo3O4]4+ cluster units. Temperature-dependent magnetic susceptibilities of 1, 2, and 7-10 were measured over the range 2-200 K and modeled by means of the spin-Hamiltonian H = - 2JS1S2 to give spin exchange coupling constants J/k of -0.60, -0.72, -1.76, -1.31, -4.80, and -1.46 K, respectively. These figures show that antiferromagnetic spin exchange coupling interactions between the magnetic, bridging M'(III) ions are very weak. These species show characteristic bands in the IR spectra at 740-820 cm-1, which can probably be assigned to ṽ(M-μ-O).
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