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  • 1
    Electronic Resource
    Electronic Resource
    Effects of alkylation upon the proton affinities of nitrogen and oxygen bases (1985)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Computational Chemistry 6 (1985), S. 168-172 
    Details
    ISSN: 0192-8651
    Keywords: Computational Chemistry and Molecular Modeling ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Computer Science
    Notes: The protonation energies of alkylated derivatives of NH3 and OH2 are calculated at the Hartree-Fock level with the split-valence 4-31G basis set. The methyl, dimethyl, and ethyl amines are studied; oxygen bases include methanol, dimethylether, and ethanol. The geometries of each molecule and its protonated analog are fully optimized. It is found that protonation leads to significant changes in the molecular structures. In particular, the bonds to the N and O atoms are substantially elongated, especially when the other atom involved is C rather than H. The calculated absolute proton affinities are somewhat larger than the experimental values. However, the differences in protonation energies of the various molecules relative to one another agree quantitatively with experiment. Replacement of one H atom of the base by a methyl group induces an increase in proton affinity of some 10 kcal/mol. If a second methyl group is added to the N or O atom, a further increment of about 70% this amount is noted. On the other hand, placement of the second C atom on the first methyl group (to form an ethyl substituent) leads to a smaller increase (∼30%). The magnitudes of these alkyl substituent effects are somewhat larger for the oxygen bases than for the amines.
    Additional Material: 4 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jcc.540060303
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  • 2
    Electronic Resource
    Electronic Resource
    Basis sets for molecular interactions. 1. Construction and tests on (HF)2 and (H2O)2 (1987)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Computational Chemistry 8 (1987), S. 663-673 
    Details
    ISSN: 0192-8651
    Keywords: Computational Chemistry and Molecular Modeling ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Computer Science
    Notes: Basis set superposition error (BSSE) remains one of the major difficulties besetting current ab initio calculations of molecular interactions. Despite the widespread notion that lowering of the BSSE to negligible magnitude requires extremely large basis sets, we show that simple modifications of basis sets of only moderate size (e.g., 6-31G**) can accomplish the same end at much reduced computational expense. These modifications include reoptimization of the orbital exponents within the framework of the relevant molecules, plus addition of a single diffuse shell of sp orbitals on nonhydrogen centers. Subsequent addition of a second set of d-functions further lowers the SCF BSSE, bringing it below 0.1 kcal/mol for both (HF)2 and (H2O)2. It is notable that addition of the latter d-functions without prior reoptimization of the valence orbitals produces the opposite effect of an increase in the BSSE. Although the MP2 BSSE is also substantially decreased by the above modifications, it appears difficult to reduce this quantity below about 0.4 kcal/mol.
    Additional Material: 6 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jcc.540080512
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  • 3
    Electronic Resource
    Electronic Resource
    Basis sets for molecular interactions. 2. Application to H3N—HF, H3N—HOH, H2O—HF, (NH3)2, and H3CH—OH2 (1987)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Computational Chemistry 8 (1987), S. 674-682 
    Details
    ISSN: 0192-8651
    Keywords: Computational Chemistry and Molecular Modeling ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Computer Science
    Notes: Modifications of the standard 6-31G** basis set as recommended in the accompanying paper are found to markedly lower the basis set superposition error (BSSE) in the title complexes, in contrast to enlargement to a triple-ζ scheme or by addition of a diffuse sp shell or a second set of d-functions without prior optimization, all of which lead to BSSE increase. After appropriate correction for correlation and superposition effects, all basis sets (with the exception of the standard 6-31G** and 6-311G** with their very large BSSE) predict the cyclic geometry of NH3 dimer to be more stable than the linear arrangement. Correlation and BSSE can shift the equilibrium intermolecular distance in H3CH-OH2 by up to 0.4 Å. Failure to correct for superposition error leads to a drastic exaggeration of both the SCF and MP2 components of the interaction energy in this complex. Much better estimates are furnished by our recommended basis sets with their smaller superposition errors.
    Additional Material: 8 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jcc.540080513
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  • 4
    Electronic Resource
    Electronic Resource
    Variation of atomic charges during proton transfer in hydrogen bonds (1994)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Computational Chemistry 15 (1994), S. 553-560 
    Details
    ISSN: 0192-8651
    Keywords: Computational Chemistry and Molecular Modeling ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Computer Science
    Notes: The point atomic charges in a number of ionic H-bonded systems are studied by ab initio calculations as functions of the proton transfer coordinate. In the proton-bound complexes of water-water, ammonia-ammonia, formamide-water, formamide-ammonia, and dimethylether-ammonia, the net atomic charges were obtained using Mulliken population analysis and from the diagonal elements of the atomic polar tensors calculated at the HF/4-31G and MP2/6-31 + G** levels. The dependence of the atomic charges upon the coordinate of the transferring proton was found to be close (within an error of 0.02 e) to a linear function for intermolecular distances in the 2.5-2.8 Å range. The obtained charge and charge flux dependencies highlight the electron redistribution during the proton transfer process and provide insights into the source of the high infrared (IR) intensities of stretching modes of N—H and O—H bonds undergoing hydrogen bonding. © 1994 by John Wiley & Sons, Inc.
    Additional Material: 8 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jcc.540150507
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  • 5
    Electronic Resource
    Electronic Resource
    Effect of nonproximate atomic substitution on excited state intramolecular proton transfer (1998)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Computational Chemistry 19 (1998), S. 129-138 
    Details
    ISSN: 0192-8651
    Keywords: ab initio ; formimidol ; H-bond ; energy barrier ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Computer Science
    Notes: The central C atom of the OCCCO skeleton of the malonaldehyde molecule is replaced by N, and the effects upon the intramolecular H-bond and the proton transfer are monitored by ab initio calculations in the ground and excited electronic states. The H-bond is weakened in the singlet and triplet states arising from n→π* excitation in both molecules, which is accompanied by a heightened barrier to proton transfer.3ππ* behaves in the same manner, but the singlet ππ* state has a stronger H-bond and lower barrier. Replacement of the central C atom by N strengthens the intramolecular H-bond. Although the proton transfer barrier in the ground state of formimidol is lower than in malonaldehyde, the barriers in all four excited states are higher in the N-analog. The latter substitution also dampens the effect of the n→π* excitation upon the H-bond and increases the excitation energies of the various states, particularly ππ*.   © 1998 John Wiley & Sons, Inc.   J Comput Chem 19: 129-138, 1998
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
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