摘要:

AbstractIn this article the quantum chemically calculated charge density distribution of 18‐crown‐6 and the K+…18‐crown‐6 complex are compared with the charge density distribution of smaller molecules and corresponding complexes which can be considered as fragments of the 18‐crown‐6 molecule. An analysis of the charge density distribution in terms of atomic charge distribution according to the stockholder recipe gives accurate rules for the transferability of the charge density distribution. This gives us the possibility to construct the charge density distribution of large molecules out of accurate large basis set results on sm

ISSN:0192-8651    

DOI:10.1002/jcc.540120102

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY

摘要:

AbstractInteraction of a rigid fragment of B‐DNA (polyanionic as well as screened by Na+cations) with the flexible tripeptide Lys‐Ala‐Ala (in bothLandDconfigurations) were investigated with the aid of an empirical potential. The potential consists of intramolecular (MM2 potential) and intermolecular (pair potential described in reference 1) parts; hence total energy is formed by intra‐ and intermolecular components. The results demonstrate that intramolecular relaxation of the peptide results in a considerable decrease in total energy. While energies of DNA complexes with L‐Lys‐L‐Ala‐L‐Ala were comparable to those with D‐Lys‐D‐Ala‐D‐Ala the respective geometries exhib

ISSN:0192-8651    

DOI:10.1002/jcc.540120103

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY

摘要:

AbstractTautomeric equilibria (mainly of the lactam‐lactim type) for a rather large number of six‐membered heterocyclic molecules are calculated by the semiempirical AM1, MNDO‐PM3, and MNDO methods. Except for compounds with adjacent pyridine‐like lone pairs both AM1 as well as MNDO‐PM3 give rather reliable predictions for relative stabilities of the various tautomeric species–comparable to quite high levelab initiocalculations. The known errors associated with MNDO in the treatment of heterocyclic tautomerism are thus largely corrected in AM1 as well as MNDO‐PM3. For 2‐hydroxypyridine‐pyrid‐2(1H)‐one the effect of self‐association is less satisfactorily described by MNDO‐PM3 than by AM1. MNDO‐PM3 calculated relative stabilities of methylated derivatives are, however, in considerably closer agreement with experimental values than those obtained by AM1. Ionization potentials, especially those for lone‐pair orbitals, are overestimated by all three semiempirical methods. MNDO‐PM3 results for nitrogen lone‐pair orbital energies are slightly better than those o

ISSN:0192-8651    

DOI:10.1002/jcc.540120104

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY

摘要:

AbstractA procedure is presented which allows a more economical representation of the potential produced by orbital charge distributions in which the orbitals are expanded in terms of a finite set of polynomial Gaussian functions. The basic idea is that the products of pairs of Gaussian basis functions, on which the charge distributions are expanded, are expressed in terms of a new basis set of optimally chosen single Gaussian functions. Such a procedure has been tested in a particular case and a few possible applications have been suggested.

ISSN:0192-8651    

DOI:10.1002/jcc.540120105

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY

摘要:

AbstractThe analysis of the hydration of NH4+and the estimation ofrelativeorabsolutefree energies of hydration by means of Monte Carlo computer simulations using different 1‐6‐12 potential functions is reported. Two electrostatic representations of NH4+(used respectively by W.L. Jorgensen and P.A. Kollman) in conjunction with two common water models (TIP3P and TIP4P) are considered. A change in relative hydration free energies of 1.7 kcal/mol is found when the NH4+models are mutated into each other in either TIP3P or TIP4P. The NH4+→ Na+mutation in both solvent models leads to similar but overestimatedrelativehydration energies of about −28.7 kcal/mol. Similarly, the NH4+annihilation significantly overestimates theabsolutefree energy of hy

ISSN:0192-8651    

DOI:10.1002/jcc.540120106

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY

摘要:

AbstractDissociation energies and potential energy surface features for the carbon clusters C2to C10are compared withab initioor experimental results for the semiempirical methods MINDO/3, MNDO, AM1, and PM3. Quite surprisingly, MINDO/3 gives a rather good account of the various structures and electronic states, unlike the other three methods. MINDO/3 tends towards systematic overestimates of binding energies, the other methods to systematic gross underestimates. Reparametrization of the diatomic parameters α, βs, and βpfor exact reproduction of the experimental data for C3results in much improved values for binding energies, but fails to correct the state splittings. Also reparametrizingUss,Upp, ζs, and ζpto reproduce theab initiolinear‐rhombic energy difference in C4results in a much improved description of the other states. For the linear structures, computed harmonic frequencies with the latter parameters are in surprisingly good agreement with experimental or correlatedab initiodata, where available; experimental values are consistently overestimated by about 40 cm−1. Other results are comparable in quality to goodab initiotreatments. The experimental IR bands at 2128 and 1892 cm−1, formerly assigned to C9, should be reassigned to linear C7. The intense 1997 cm−1feature almost certainly belongs to C9; bands at 1952 and 1197 cm−1both belong to linear C6. Tentative assignments of bands in the 1600–1850 cm−1region to various cyclic structures of C6, C8, and C10have been made. As such, this suggests a new and promising procedure for the theoretical study of large molecules in general, and of large clus

ISSN:0192-8651    

DOI:10.1002/jcc.540120107

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY

摘要:

AbstractA Brownian dynamics simulation program for the computation of transport properties of charged polarizable macromolecules is presented. The program contains routines for simulating polymer dynamics in a gel network as well as in a quiescent fluid or in a shear field. An electric field can also be included to simulate e.g., gel electrophoresis or electric dichroism. Vector algorithms have been used where possible to obtain maximum efficiency on a vector processor. The length of the vector registers and the effect of computer memory organization have also been considered. On an IBM 3090‐150S VF, more than 1.5 · 106steps are executed in one cpu hour for a chain with 128 subunits diffusing in a rigid gel consisting of 8828 bea

ISSN:0192-8651    

DOI:10.1002/jcc.540120108

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY

摘要:

AbstractA molecular mechanics force field was developed for systems bearing the NCO unit on the basis of 6‐31G* and 4‐21G “ab initio” calculations with full optimization of the geometry and experimental heats of formation. The parameters used, which implicitly included the anomeric effect, provided good geometric and energetic results, both for the compounds on which the parametrization was based and for others on which the validity of the predictions

ISSN:0192-8651    

DOI:10.1002/jcc.540120109

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY

摘要:

AbstractAtomic multipoles as defined by current methods generally do not account for forces in molecules that arise from external electrostatic fields. It is pointed out that such forces and the electrostatic potential that the molecule itself generates are both determined by the molecular multipolar tensors. The latter constitute therefore the fundamental molecular constants that determine the molecular electrostatics apart from polarization. In general the multipolar tensors include contributions from the atomic multipoles and their fluxes. In planar molecules, however, the perpendicular charge flux is zero by symmetry. This gives rise to a (previously introduced) formalism that extracts analytical, force‐related, atomic multipoles from the molecular multipolar tensors. This formalism is extended in this work to include force‐related (FR) atomic quadrupoles and octupoles in planar molecules. The properties of the FR atomic multipoles, including their perpendicular fluxes, are discussed and some formal theoretical and computational advantages that characterize them are indicated. As an example, the electrostatics of OCS, including the molecular electrostatic potential and the forces on the nuclei due to an external point charge, is discus

ISSN:0192-8651    

DOI:10.1002/jcc.540120110

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY

摘要:

AbstractA vectorized computer code is developed for the enumeration of walks through the matrix power method for directed graphs. Application of this code to several graphs is considered. It is shown that the coefficients in the generating functions for signed graphs are much smaller in magnitude. It is shown that self‐avoiding walks on some graphs can be enumerated as a linear combination of walk GFs of directed paths and rooted‐directed pa

ISSN:0192-8651    

DOI:10.1002/jcc.540120111

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY