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  • 1
    ISSN: 0947-6539
    Keywords: ab initio calculations ; copper compounds ; organometallic compounds ; tetrahydroborato ligand ; theoretical chemistry ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: All-electron ab initio calculations (MP4/MP2 level) have been performed on [Cu(BH4)(PH3)n] (n = 1, 2, 3) complexes. Full-geometry optimizations were carried out in each case, and the stationary points were characterized by the diagonalization of the analytically calculated Hessian matrix. The η2 coordination mode, with a tetrahedral arrangement around the copper atom, is the most stable structure for n = 2, while for n = 3 a strongly nonlinear η1 coordination mode is preferred. These results are in agreement with the experimental data available on related complexes. For n = 1, for which there is no experimental data, the η3 structure turns out to be the most stable. The energy differences associated with some changes in the coordination mode (η1 → η2 for n = 3 and η3 → η2 for n = 1) are small. Finally, a mechanism for the exchange between terminal and bridging hydrogen atoms is proposed for each complex under study.
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  • 2
    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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  • 3
    ISSN: 0947-6539
    Keywords: ab initio calculations ; dihydrogen complexes ; isotope effects ; NMR spectroscopy ; osmium complexes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Reaction of complex [OsHCl-(CO)(PiPr3)2] (1) with hydrogen gives the trans-hydridodihydrogen complex [OsH-Cl(n2-H2)(CO)(PiPr3)2] (2). The H-H distance in the dihydrogen ligand, determined by variable-temperature 1H T1 measurements, is 0.8 Å. The fast-spinning nature of the dihydrogen ligand of 2 has been deduced by evaluating the deuterium quadrupole coupling constant for the n2-D2 ligand of [OsDCl(n2-D2)(CO)(PiPr3)2] ([D3]2). Measurements of the equilibrium constants for formation of 2 (K) give ΔH0 = - 14.1 ± 0.5 kcal mol-1 and ΔS0 = - 30 ±1 e.u. An equilibrium isotope effect Kd/K of 2.8 is found for this reaction. The activation parameters for the H2 loss from 2 are ΔH
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  • 4
    ISSN: 0947-6539
    Keywords: ab initio calculations ; rotamers ; stereoisomers ; sulfane oxides ; torsional potentials ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Ab initio MO calculations (MP 2/6-311 G**//HF/6-311 G**) have been performed for several isomers (including rotamers) of Me2S3O and Me2S3O2. MeS(O)SSMe exists as five rotamers; the most stable form (1a) has a helical backbone CSSSC with S-S bond lengths of 206.0 (SIISII) and 212.4 pm (SIISII). The most stable rotamer of MeS-S(O)SMe (2a), is less stable than 1a by 10.7 kJ mol-1; it is of Cs, symmetry, while a rotamer of Cs symmetry (2b) is less stable than 2a by only 1.4 kJ mol-1. Both 2a and 2b are stabilized by O ⃛ H hydrogen bonds. The S-S bond lengths of 2a are 210.0 and 212.0 pm; the CSSSC chain is not helical (CSSS torsion angles 166.3 and -75.4°). The 1,3-dioxide MeS(O)-SS(O)Me (3) has two equivalent chiral centers and exists as diastereomers. The most stable isomer 3a (RR)/(SS) is of C2 symmetry with methyl groups trans to each other; the SO bonds form an angle of about 90°. The meso form 3b is less stable than 3a by 17.2 kJ mol-1 and the rotamers 3c and 3d are less stable by 25.6 kJ mol-1 and 28.4 kJ mol-1, respectively. The trisulfane-1.2-dioxide MeS(O)-S(O)SMe has two nonequivalent chiral centers and exists as five isomers. The most stable form, the (RS)/(SR) form 4a, is less stable than 3a by 21.4 kJ mol-1 and is characterized by SS bonds of 220.9 (SIIISIII) and 208.3 pm (SIIISII). The rotamer 4b is less stable by 5.9 kJ mol-1. The isomers 4c, 4d, and 4e are all of (SS)/(RR) configuration and are less stable than 4a by 6.3, 12.7, and 12.0 kJ mol-1. For comparison, ab initio MO calculations (HF/6-311 G*) for tBuS(O)SS(O)-tBu yielded two diastereomers of practically identically energy that both contain helical CSSSC backbones. The (RS) form is less stable than the (RR)/(SS) form by 1.8 kJ mol-1.
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  • 5
    ISSN: 0947-6539
    Keywords: ab initio calculations ; cobalt ; planar carbon ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Quantum-chemical methods have been employed to study the nature of stabilization in dinuclear cobalt complexes of the general formula [{(C5H5)Co}2(μ-CR12BCBR2R3)] (6) as well as the “antivan't Hoff-Le Bel” configuration of the planar tetracoordinate carbon (ptC) atom of the bridging diborylcarbene ligand 9. Extended Hückel and ab initio Hartree-Fock calculations have been carried out for the model compounds 6b (R1 = R2 = R3 = H) and 6c (R1 = R2 = H; R3 = C6H5). Ab initio electron deformation density maps and natural population analysis calculations show that complexes 6 are stabilized through push-pull effects by which the ptC experiences π electron density delocalization and σ electron density accumulation. The calculated electronic configuration of the ptC in the free ligand 9b is σ2.978π1.501, and in 6b σ3.944π1.356. Electron density donation from one cobalt atom to an aryl group on the bridging ligand further contributes to the stabilization of the complexes 6.
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  • 6
    ISSN: 0947-6539
    Keywords: ab initio calculations ; enzymatic catalysis ; enzyme models ; mechanistic studies ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Ab initio quantum mechanical model calculations were used in studying the acylation and deacylation steps of the catalytic mechanism of aspartylglucosaminidase (AGA). AGA catalyses the hydrolysis of an amide linkage between oligosaccharide and asparagine by utilising an N-terminal threonine as a catalytic amino acid. Results are reported for the model enzyme reaction at the MP2/6-31+G*//HF/6-31 + G* + ΔZPE level. Contribution of aqueous solvation to the relative energies was estimated by using the continuum solvation model of Tomasi. The serine protease-like catalytic mechanism was found to be feasible for AGA. The protonated α-amino group of the substrate of AGA was suggested to enhance the catalysis by stabilising the anionic oxygen of the substrate, which is formed in the reaction, and by lowering the pKa of the nucleophilic oxygen of the N-terminal threonine. Finally, the similarities in the catalytic mechanisms of AGA and other amidohydrolases were discussed.
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  • 7
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 89-98 
    ISSN: 0947-6539
    Keywords: ab initio calculations ; Cambridge structural database ; fluorine compounds ; hydrogen bonds ; protein data bank ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Statistical analysis of structural data and detailed inspection of individual crystal structures culled from the Cambridge Structural Database and the Brookhaven Protein Data Bank show that covalently bound fluorine (in contrast to anionic fluoride) hardly ever acts as a hydrogen-bond acceptor. The weakness of covalently bound fluorine as hydrogen-bond acceptor is backed by results of new molecular orbital calculations on model systems using ab initio intermolecular perturbation theory (IMPT), and is in accord with results of other physicochemical studies and with the physical properties of fluorinated organic compounds. Factors influencing the strength of hydrogen bonding in extended systems are discussed.
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  • 8
    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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  • 9
    ISSN: 0947-6539
    Keywords: ab initio calculations ; clusters ; electronic structure ; gallium ; semiempirical calculations ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We present idealised geometrical models for the α-Ga and β-Ga structures based on two differently distorted, corrugated 36 nets. We investigated the structural stability of two sets of two-dimensional and three-dimensional model structures consisting of these corrugated nets as a function of the net puckering. We used the simple tight-binding (TB) Hückel model with the structural energy difference theorem and the advanced full-potential linear muffin-tin orbital (FP-LMTO) method in the framework of local-density functional theory. Both methods show the existence of an optimum puckering angle that corresponds well to the situation in the experimental α-Ga structure. The geometrical model of the α-Ga structure follows the building principle of terminally coordinated deltahedral clusters, extended to two-dimensional structures. The chemical bonding in α-Ga is interpreted in terms of multicentre bonding within the corrugated 36 nets and two-electron-two-centre bonds connecting these nets.
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  • 10
    ISSN: 0947-6539
    Keywords: ab initio calculations ; gold ; isolobal relationship ; sulfonium trication ; superelectrophiles ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The pentahydridosulfonium trication SH3+5 was found by ab initio MP2/ 6-31G** and QCISD(T)/6-311G** levels of calculation to be a stable minimum with Cs symmetric structure 1. It is isostructural with the parent pentacoordinate carbonium ion CH+5. Structure 1 resembles a complex between SH3+3 and molecular hydrogen forming a 2e-3c bond. This structure involving a pentaco-ordinate sulfur atom is unprecedented. Rotation of the H2 unit around the pseudo-C3 axis of SH3 is facile. The structure of SF3+5 was also calculated and compared with that of 1. In addition, our studies at the ab initio MP2/6-31G** level showed that the parent pentacoordinate oxonium trication OH3+5 is not a minimum on its potential energy surface, and dissociation into OH2+4 and H+ occurs upon optimization.
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