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  • 1
    ISSN: 1432-2234
    Keywords: Oxime nitrogen inversion ; Formaldoxime
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Semi empirical CNDO and “ab initio” methods are applied to the analysis of syn-anti isomerization mechanism of formaldoxime. Semi empirical calculations are carried out with complete geometry optimization and lead to predicted inversion barrier equal to 37.5 kcal/mole. A bicentric partitioning of the total energy and the expression of the density matrix in hybride basis set reveal the chemical origin of the shape of the potential line.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1432-2234
    Keywords: Key words: Solvation model ; Solvents ; organic ; Electrostatic effects ; Partial charges ; class IV ; Surface tensions ; atomic
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract. The SM5.4 quantum mechanical solvation model has been extended to calculate free energies of solvation in virtually any organic solvent. Electrostatics and solute-solvent polarization are included self-consistently by the generalized Born equation with class IV charges, and first-solvation-shell effects are modeled in terms of solvent-accessible surface areas that depend on solute geometries and four solvent descriptors. The inclusion of solvent properties in the first-solvation-shell term provides a model that predicts accurate solvation free energies in any solvent for which those properties are known. The model was developed using 1786 experimentally measured solvation free energies for 206 solutes in one or more of 90 solvents. Parameters have been obtained for solutes containing H, C, N, O, F, S, Cl, Br, and I, and the solutes used for parameterization span a wide range of organic functional groups. Solvents used in the parameterization contain H, C, N, O, F, P, S, Cl, Br, and I and include the most common organic solvents. Two general parameterizations are presented here, one for use with the AM1 Hamiltonian (SM5.4/AM1) and one for use with the PM3 Hamiltonian (SM5.4/PM3). In each case, one parameter is specially re-optimized for benzene and toluene to reduce systematic errors for these solvents. Chloroform is also treated with special parameters. The final mean unsigned error for both the SM5.4/AM1 and SM5.4/PM3 parameterizations is less than 0.5kcalmol−1 over the entire data set of 1786 free energies of solvation in 90 organic solvents.
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  • 3
    ISSN: 1432-2234
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract. We present eight new parameterizations of the SM5.42R solvation model: in particular we present parameterizations for HF/MIDI!, HF/6-31G*, HF/6-31+G*, HF/cc-pVDZ, AM1, PM3, BPW91/MIDI!, and B3LYP/MIDI!. Two of the new cases are parameterized using the reaction-field operator presented previously, and six of the new cases are parameterized with a simplified reaction-field operator; results obtained by the two methods are compared for selected examples. For a training set of 2135 data for 275 neutral solutes containing H, C, N, O, F, S, P, Cl, Br, and I in 91 solvents (water and 90 nonaqueous solvents), seven of the eight new parameterizations give mean unsigned errors in the range 0.43–0.46 kcal/mol, and the eighth – for a basis set containing diffuse functions – gives a mean unsigned error of 0.53 kcal/mol. The mean unsigned error for 49 ionic solutes (containing the same elements) in water is 3.5–3.9 kcal/mol for the Hartree–Fock, Becke–Perdew–Wang-1991 and Becke three-parameter Lee–Yang–Parr cases and 4.1 and 4.0 kcal/mol for parameterized model 3 and Austin model 1, respectively. The methods are tested for sensitivity of solvation free energies to geometry and for predicting partition coefficients of carbonates, which were not included in the training set.
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  • 4
    ISSN: 1432-2234
    Keywords: Oximes ; Photochemical isomerism
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Photochemical syn-anti isomerism of oximes is approached by both semi-empirical andab initio calculations. The geometry and the energy of different intermediate species are evaluated by optimization. The results agree with available experimental data. A mechanism is proposed which involves the peculiar form of the energy surface in the triplet state.
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  • 5
    ISSN: 1432-2234
    Keywords: Morse theory ; Potential surfaces ; Magnesium clusters
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Summary The Morse inequalities linking the critical points of a potential function on the whole configuration space and its restrictions to either planar or linear configurations are derived from the Morse theory in its equivariant form. Brute potential functions arising from standard models of quantum chemistry need eventually morsification which can be achieved without altering the main chemical significances of the potential. Illustrative applications follow in the case of magnesium clusters.
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  • 6
    ISSN: 1572-8897
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mathematics
    Notes: Abstract The whole sets of critical points of analytical functions corresponding to the long‐range two‐body interaction between two, three and four dipole vectors set at the vertices of several polygons are determined using topology theorems. Betti numbers associated to the configuration spaces are first obtained, then stationary points are located via an analytical gradient method till the Morse inequalities are checked.
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  • 7
    ISSN: 0192-8651
    Keywords: Computational Chemistry and Molecular Modeling ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Computer Science
    Notes: We present improved algorithms for the SMx (x = 1, 1a, 2, 3) solvation models presented previously [see the overview in C. J. Cramer and D. G. Truhlar, J. Comp.-Aided Mol. Design, 6, 629 (1992)]. These models estimate the free energy of solvation by augmenting a semiempirical Hartree-Fock calculation on the solute with the generalized Born (GB) model for electric polarization of the solvent and a surface tension term based on solvent-accessible surface area. This article presents three improvements in the algorithms used to carry out such calculations, namely (1) an analytical accessible surface area algorithm, (2) a more efficient radial integration scheme for the dielectric screening computation in the GB model, and (3) a damping algorithm for updating the GB contribution to the Fock update during the iterations to achieve a self-consistent field. Improvements (1) and (2) decrease the computer time, and improvement (3) leads to more stable convergence. Improvement (2) removes a small systematic numerical error that was explicitly absorbed into the parameterization in the SMx models. Therefore, we have adjusted the parameters for one of the previous models to yield essentially identical performance as was obtained originally while simultaneously taking advantage of improvement (2). The resulting model is called SM2.1. The fact that we obtain similar results after removing the systematic quadrature bias attests to the robustness of the original parameterization. © 1995 by John Wiley & Sons, Inc.
    Additional Material: 13 Ill.
    Type of Medium: Electronic Resource
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  • 8
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The most efficient optimization methods implemented in the semiempirical package AMPAC are presented. They concern the minimization of the energy or of the gradient norm by either pseudo-Newton or quadratic procedures, the search for transition states, and the intrinsic reaction coordinate in conjunction with variational transition-state theories. Nonlocal methods such as simulated annealing are also introduced. © 1992 John Wiley & Sons, Inc.
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