摘要:

AbstractAb initioSCF‐LCAO‐MO computations using a minimal basis set have been carried out on the acetylcholine ion in different conformations. The rotational barrier around the C9‐C8‐O1‐C2 dihedral angle (τ1) has been also calculated: the curve presents a broad minimum. SCF calculations have been carried out on the system Ach‐water at 468 different separations and/or orientations; the interaction energies were then fitted to a pair potential function used in Monte Carlo (MC) simulations of Ach water solution. The rdfs indicate a global first hydration shell with about 38 waters. Details about the hydration shell are presented. The overall MC results indicate that for the Ach ion in water solution at room temperature relative free rotation is possible around τ1, and that Ach is well represented as a large positively charged hydrated globe with a small mobile tail free of strong interactions with the surrounding wa

ISSN:0192-8651    

DOI:10.1002/jcc.540090102

出版商:John Wiley&Sons, Inc.    

年代:1988

数据来源: WILEY

摘要:

AbstractThe positions and orientations of water molecules in violuric acid crystals have been determined with the Metropolis Monte Carlo method. The interaction potentials between water and violuric acid needed in the simulation have been developed usingab initiocalculations corrected for the basis set superposition error with the C.P. technique and fitted to atom‐atom pairwise potentials. The agreement between experimental data and simulation results is good, proving that the interaction potentials can be used with confidenc

ISSN:0192-8651    

DOI:10.1002/jcc.540090103

出版商:John Wiley&Sons, Inc.    

年代:1988

数据来源: WILEY

摘要:

AbstractThis article examines the numerical estimation of molecular surface areas within the model of overlapping atomic spheres. One has the choice of either basing the estimate on all elements that contribute to the surface, or of ignoring systematically some elements in the interatomic clefts. It is argued that the second choice, even though more approximate, implicitly improves on the model and is to be preferred. Since surface areas are not measurable, the demonstration is unavoidably roundabout, relying mostly on correlation analysis. Among the regressors occur two compounded parameters. One, ratio of the surface area of the equivalent sphere to the surface area, is interpreted as a measure of molecular globularity. It reflects the molecular axis‐ratio and surface convolution. The other, ratio of the surface area to the volume, is interpreted as a measure of the global congestion of a chemical residue. Together with a measure of the local congestion at the point of attachment, it affects the steric hindrance that a residue offers. The relation between the surface area and the number of valence electrons is also discusse

ISSN:0192-8651    

DOI:10.1002/jcc.540090104

出版商:John Wiley&Sons, Inc.    

年代:1988

数据来源: WILEY

摘要:

AbstractMolecular structures of 26 organosilicon compounds have been optimized usingab initiocalculations at the 3‐21G* (modified) level. From these optimized structures, the internal coordinates have been deformed and the variation of the total molecular energy has been studied. Parameters for stretching and bending deformations are reported herein. The bending potential for the SiOSi bond which has an unusual flexibility is also included. Nonbonding interactions are described in terms of steric and electrostatic potentials. For systems which do not include bond resonance effects, torsional behavior is well described by steric potentials with van der Waals radii 20% larger than the previous values and simple electrostatic potential (monopole‐monopole) with net atomic charges obtained fromab initioor Extended Huckel Theory calculations. The nonbonding potentials, as defined in this study, have an advantage in that they allow for the computation of torsional barriers without torsional potentials, in the case of single bonds where no additional electronic effects interfere. As an example, it is shown that no torsional potentials are necessary to estimate the torsional barriers in the case of ethane. The newly defined potentials are used to study the torsional barrier in hexamethyldisiloxane and the conformation of octamethylcyclotetrasiloxane (D4). The most stable calculated conformation of D4, coincides with the experimentally determined structure. This study shows that the most stable conformation is determined by the steric repulsion of methyl

ISSN:0192-8651    

DOI:10.1002/jcc.540090105

出版商:John Wiley&Sons, Inc.    

年代:1988

数据来源: WILEY

摘要:

AbstractSINDO1 calculations are presented for ground state geometries, heats of formation, ionization potentials and dipole moments. These calculations are based on a new parametrization of SINDO1 for second‐row elements which features inclusion of 3dorbitals and zero point energies. The comparison shows an improvement over MNDO, especially for hypervalent phosphorus compound

ISSN:0192-8651    

DOI:10.1002/jcc.540090106

出版商:John Wiley&Sons, Inc.    

年代:1988

数据来源: WILEY

摘要:

AbstractSINDO1 calculations are presented for ground state geometries, heats of formation, ionization potentials and dipole moments of silicon, aluminum and magnesium compounds. These calculations are based on a new parametrization of SINDO1 for second‐row elements. Important features are the inclusion of 3dorbitals and the explicit evaluation of zero point energies. A comparison with MNDO data is presente

ISSN:0192-8651    

DOI:10.1002/jcc.540090107

出版商:John Wiley&Sons, Inc.    

年代:1988

数据来源: WILEY

摘要:

AbstractThe multidimensional potential energy surface for a soluble analog of a chiral phase is computed with MM2 and MNDO. Minimum energy conformations are located. The minimum energy reaction pathway between these forms is located, and the templating ability of these phases is described.

ISSN:0192-8651    

DOI:10.1002/jcc.540090108

出版商:John Wiley&Sons, Inc.    

年代:1988

数据来源: WILEY

摘要:

AbstractThe conjugated system (E)‐tiglaldoxime is the simplest example of a perillartine analog which exhibits sweetness with a taste potency greater than sucrose with almost no bitter aftertaste. In previous studies, the structure of this biologically active compound has been assumed to be planar with the CC double bondtransto the CN bond of the oxime moiety. In this article a conformational analysis of this molecule is reported. The results indicate that, although thetransconformer of the planar molecule is indeed the global minimum, other conformers lie within a few kilocalories of this minimum. Hence, other accessible conformations may be available for interaction with the receptor and, therefore, may be biologically active. The structural parameters obtained for this conjugatedsyn‐oxime are nearly identical to those of (E)‐acetaldoxime. This fact has implications for the transferability of these parameters to the more complicated perillartin

ISSN:0192-8651    

DOI:10.1002/jcc.540090109

出版商:John Wiley&Sons, Inc.    

年代:1988

数据来源: WILEY

摘要:

AbstractIt is shown how standard computer programs for calculating the equilibrium geometry of a molecule may be adapted to yield floating functions. These functions satisfy the Hellmann–Feynman theorem and so lead to simple electrostatic interpretations of intramolecular forces and vibrations. The theory is illustrated by detailed calculations for the water molecul

ISSN:0192-8651    

DOI:10.1002/jcc.540090110

出版商:John Wiley&Sons, Inc.    

年代:1988

数据来源: WILEY

摘要:

AbstractIn an earlier article8the need was demonstrated for atomic physicochemical properties for three dimensional structure directed quantitative structure‐activity relationships, and it was shown how atomic parameters can be developed for successfully evaluating the molecular octanol‐water partition coefficient, which is a measure of hydrophobicity. In this work we report more refined atomic values of octanol‐water partition coefficients derived from nearly twice the number of compounds. Carbon, hydrogen, oxygen, nitrogen, sulfur and halogens are divided into 110 atom types of which 94 atomic values are evaluated from 830 molecules by least squares. These values gave a standard deviation of 0.470 and a correlation coefficient of 0.931. These parameters predicted the octanol‐water partition coefficient of 125 compounds with a standard deviation of 0.520 and a correlation coefficient of 0.870. There is only a correlation coefficient of 0.432 between the atomic octanol‐water partition coefficients and the atomic contributions to molar refractivity over the 93 atom types used for both the properties. This suggests that both parameters can be used simultaneously to model intermolecular interactions. We evaluated the CNDO/2 gross atomic charge distribution over several molecules to check the validity of our classification. We found that the charge density on the heteroatoms in conjugated systems is strongly affected by the presence of similar atoms in the conjugation which suggests it should be incorporated as a separate parameter in evaluating the partition co

ISSN:0192-8651    

DOI:10.1002/jcc.540090111

出版商:John Wiley&Sons, Inc.    

年代:1988

数据来源: WILEY