摘要:

AbstractA free energy barrier ΔF≠= 174.2 kJ/mol for the self‐exchange electron transfer reaction model Fe+/Fe2+in water has been calculated by combining Monte Carlo simulations and the statistical perturbation theory. We have shown that, even for those electron transfer reactions that present a very high free energy barrier of activation, the free energy curve behaves parabolically versus the reaction coordinate, which justifies the quadratic expression for the activation free energy done by Ma

ISSN:0192-8651    

DOI:10.1002/jcc.540121002

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY

摘要:

AbstractSlater type orbital (STO) basis sets for the atoms Sc‐Zn have been derived using a technique based on the distance between subspaces. The accuracy for several properties of these basis sets has been tested. Basis sets studied are of both single‐ and double‐zeta sizes, although this technique can be generalized for any size. Uniform quality criteria through the series of atoms Sc‐Zn are difficulty to establish due to the varying number ofdelectrons. A comparative study at the atomic level of the quality of STO basis sets (both the two new basis sets and Clementi's basis sets) for the first‐row transition elements has been carried out. Results show that the new basis sets provide better simulation for several properties. Molecular calculations on compounds with these atoms using a Gaussian expansion fitted according to the new values of optimized STOs are also included. The results obtained are similar to those reported when STO‐3G basis

ISSN:0192-8651    

DOI:10.1002/jcc.540121003

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY

摘要:

AbstractThe octanol‐water partition coefficients (Log P) of tripeptide diastereoisomers have been calculated by our recently developed method for predicting Log P from the results of AM1 semiempirical calculations. This method can distinguish different diastereoisomers. The differences arise from variations in molecular conformation, which lead to significant differences in the dipole moment and ovality of the different diastereoisomer

ISSN:0192-8651    

DOI:10.1002/jcc.540121004

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY

摘要:

AbstractWe have developed a method for building up accurate electrostatic models for polypeptides, based on a distributed multipole representation of the SCF charge densities for the dipeptides (CH3.CO.NH.CHR.CO.NH.CH3) of the naturally occurring amino acids. It is based on the observation that each peptide residue has almost the correct formal charge (0, ±1). We find that the variations in the backbone charge distributions (excluding proline) with sidechain have a negligible effect on the predicted electrostatic potential around the residue. However changes in the atomic multipoles with the torsion angles (ϕ, ψ, χ) are more significant, and may need to be taken into account if electrostatic potential close to the residue is required to high accuracy. This type of DMA peptide library provides more accurate, more theoretically based, estimates of the electrostatic potential around polypeptides than current mod

ISSN:0192-8651    

DOI:10.1002/jcc.540121005

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY

摘要:

AbstractThe purpose of this study is to design a technique that generates a variable van der Waals‐like surface for any desired charge density value of a molecule. The method is based on the recognition that for each fixed density value, within a wide range, certain shape features of the electron density contours are approximately invariant under configuration rearrangements, such as bending and stretchings in polyatomic molecules. These invariants appear in the form of spherical regions in space around the nuclei, where no density contours of a given level set value enter. These regions, the so‐called invariant cores, behave as approximately transferable quasi‐atomic spherical surfaces. We have evaluated these cores in molecules, and also approximated them in terms of the electron density of isolated atoms. The properties of these quasi‐atomic surfaces allow one to design a scale of variable atomic radii based on the size of the transferable isodensity spheres. In this work we provide a detailed description of the dependence of the diameters of these spheres on the electron density values. We show that, by using this scale of radii, it is possible to design fused‐sphere molecular models that closely mimic isodensity surfaces for a whole range of density values. The results discussed here allow one to relate molecular size with charge density. We propose the use of these scales for the construction of more realistic van der Waals model surfaces when studying molecular shape in the context of computer‐assisted molecul

ISSN:0192-8651    

DOI:10.1002/jcc.540121006

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY

摘要:

AbstractThe products of the rearrangement of ethylnitrene are ethylideneimine via a 1,2 hydrogen shift,N‐methyl methyleneimine via a 1,2 methyl shift, and the corresponding aziridine by cyclization. The three reactions have been studied by the semiempirical MNDO method. The charge transfer effects have been investigated. Theoretically calculated vibrational frequencies and kinetic properties have been used in calculating macroscopic thermodynamic and kinetic properties. The entropies of activation, the high pressure rate constants,k∞, and the high pressure Arrhenius parameters,A∞andE∞, have been computed. The kinetic isotope effects have also been

ISSN:0192-8651    

DOI:10.1002/jcc.540121007

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY

摘要:

AbstractUsing the Löwdin alpha‐function method in which displaced orbitals are expanded in spherical harmonics, two‐center, two‐electron repulsion integrals of the Coulomb, hybrid, and exchange type are done analytically using Slater‐type orbitals. Computer algebra and integer arithmetic are used to obtain analytic results and avoid cancellation errors by the generation of rational matrix elements forC,E, andFmatrices that are used to express the α‐function. The formula for the exchange integral is kept simple by reversing the order of integration over each part of a split quadrant. Only two basic integrals are used that are first efficiently evaluated by using look‐up tables and then us

ISSN:0192-8651    

DOI:10.1002/jcc.540121008

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY

摘要:

AbstractGiven a reaction mechanism we show how a symbolic computation approach can be used to develop the kinetic equations by identifying the reaction scheme with an equivalent matrix. Our method is also applicable in cases where the stoichiometric matrix approach fails. The specific algorithm that is written applies to schemes where individual reactions are at most ternary, but the way to generalize the procedure is also discussed. By using symbolic computing it is possible to determine general properties of the system. We demonstrate this by showing how to use the matrix to determine the system's conservation laws, which in turn can be used to reduce the number of equations in the system. As another application it is shown how to determine some of the species which have a zero equilibrium state. To illustrate the procedure, example reaction schemes are investigated.

ISSN:0192-8651    

DOI:10.1002/jcc.540121009

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY

摘要:

AbstractWe have carried out free energy perturbation calculations on the relative solvation free energy ofcis‐ andtrans‐N‐methyl‐acetamide (NMA). Experimentally, the solvation free energy difference has been found to be near zero. Using 6‐31G*ab initioelectrostatic potential derived charges for both thecisandtransconformations, we calculate a solvation free energy difference of 0.1 ± 0.1 kcal/mol. Using the 6‐31G* charges derived for thetransconformation for both thecisandtransmodels leads to a solvation free energy difference of 0.9 ± 0.1 kcal/mol, compared to the value of 2.2 kcal/mol determined for the OPLS model

ISSN:0192-8651    

DOI:10.1002/jcc.540121010

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY

摘要:

AbstractA finite element method approach for solving the three‐dimensional Schrödinger equation expressed in hyperspherical coordinates is applied to the calculation of rovibrational states of H2O and D2O. Comparisons to experimental values and other theoretical calculations are offer

ISSN:0192-8651    

DOI:10.1002/jcc.540121011

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY