摘要:

AbstractProton affinities (PAs) of 2‐, 3‐, and 4‐monosubstituted phenolates in the gas phase are calculated using the MINDO/3, MNDO, and AM1 methods. The following substituents are considered: H, F, Cl (only MNDO and AM1), CN, CH3, OH, NO2, OCH3, COOH, and NH2. The results are compared with experimental values (high‐pressure mass spectroscopy) at 600 K. The MINDO/3 method overestimates the experimental PAs with a mean percentual error of ca. 4.3%. The MNDO results agree much better with the experimental values and are only ca. 1.3% too high. However, the best agreement is observed for the AM1 method, where the mean percentual error is only ca. 0.69% (in absolute value). The greatest disagreement is found for 2‐hydroxyphenolate (ca. +2.0%) and 2‐carboxyphenolate (ca. +2.7%). The PA of 2‐nitrophenolate is underestimated by ca. 1.6%. Correlations between the calculated proton affinities on the one hand, charge densities on the acid H atom of the phenols, charges on the O atom in the phenolate ions, and substituent parameters on the other hand are studied. Particularly good linear relationships are found for the 3‐ and 4‐monosubstituted compounds within the AM1 formlism. For the 2‐substituted phenolates, only the AM1 method is satifactory. As a general conclusion, the AM1 method appears to be distinctly superior to MNDO for the calculation of PAs, and definitely

ISSN:0192-8651    

DOI:10.1002/jcc.540110302

出版商:John Wiley&Sons, Inc.    

年代:1990

数据来源: WILEY

摘要:

AbstractThe adsorption of methane, ethane, propane,n‐butane, ethylene, propene, 1‐butene, 1,3‐butadiene, acetylene, benzene, toluene, naphthalene, anthracene, and pyrene on the basal plane of graphite has been studied by means of a semiempirical method proposed by Fraga based on a 1/Rexpansion of atom‐atom pair potentials. The energies of interaction obtained agree reasonably well with experimental data and statistical calculations. The orientation of the adsorbed molecules relative to the graphite surface rendered by Fraga's method improves the picture provided by other calculations with empirical potentials. These results display Fraga's method as an economical alternative for the study of physisorption of hydrocarbons on the graphite

ISSN:0192-8651    

DOI:10.1002/jcc.540110303

出版商:John Wiley&Sons, Inc.    

年代:1990

数据来源: WILEY

摘要:

AbstractThe dependence of net atomic charges, as derived from least‐squares fitting to electrostatic potentials, on molecular orientation and potential site location is critically examined. A unique rotationally invariant algorithm, in which the charges may be constrained to reproduce the molecular dipole moment, is presented and the results for several basis sets compared to previous work. Significant improvements in dipole moments derived from the unconstrained charges are note

ISSN:0192-8651    

DOI:10.1002/jcc.540110304

出版商:John Wiley&Sons, Inc.    

年代:1990

数据来源: WILEY

摘要:

AbstractA common approximation used inab initiomolecular orbital calculations assumes that the changes in energy due to use of a larger basis set (as in the inclusion of polarization orbitals) and to allowance for electron correlation are additives. Thus, small basis sets may be used for correlatedab initiocalculations, resulting in a great savings in time. While this approach has been shown to perform acceptably in some cases, a systematic study has not been published. We examined 35 species and computed the heats of reaction for 15 processes at the 6‐31G level of theory using polarization and correlation correction up to the MP4 level. The same items were then treated by an additive procedure to attain the same level of theory. We also examined the conrotrary ring opening of cyclobutene. Our results indicate that the errors do cancel to some extent but that the uncertainty associated with open shell species is significantly larger than closed shell systems. This calls into question the use of the procedure in predicting the properties of transition state

ISSN:0192-8651    

DOI:10.1002/jcc.540110305

出版商:John Wiley&Sons, Inc.    

年代:1990

数据来源: WILEY

摘要:

AbstractWe have undertaken an energy minimization study of the binding of a small ligand, water, to amino acid side‐chains. These calculations have been performed on a Meiko Computing surface containing a small number of transputers and on a larger transputer array installed at Edinburgh University. The aim of this study is twofold. First, we wished to compare these potential energy maps for a given side‐chain as a function of the local secondary structure and also of the neighboring residue types. Secondly, the energy maps are found to be in good agreement with experimental distributions on the binding of water molecules to amino acid side‐chains. The use of our in‐house Meiko computing surface increases the speed of these calculations by a factor of about 25 over a VAX 11/750. With larger arrays of transputers, such as that at Edinburgh “concurrent” Supercomputer Project, we achieve increases in speed of over 200 such that these energy maps can be calculated at interac

ISSN:0192-8651    

DOI:10.1002/jcc.540110306

出版商:John Wiley&Sons, Inc.    

年代:1990

数据来源: WILEY

摘要:

AbstractEmpirical force‐field calculations were carried out before and after metal‐complex formation for a cobalt(III)‐complex system with a series of aliphatic amines. The limit of the strain‐energy differences in metal‐complex formation is evaluated by comparing the calculated data with experimental results. A criterion for the synthesizability of unknown metal complexes was derived from these findings. The strain energy of the ion pair was also calculated for some of the complexes. The results suggest that the ion‐pair formation effectively contributes to their st

ISSN:0192-8651    

DOI:10.1002/jcc.540110307

出版商:John Wiley&Sons, Inc.    

年代:1990

数据来源: WILEY

摘要:

AbstractSimplified formulas for first and second derivatives of the internal coordinates with respect to Cartesian coordinates are reported for linear and out‐of‐plane bending motion. They are expressed in a local coordinate system and then rotated to a space‐fixed Cartesian coordinate system. For linear motion the important points are: (1) the invariance of the energy with respect to translational and rotational coordinate transformations and (2) the presentation of derivatives of the energy in terms of (α − π)2rather than (α − π), where α is the bond angle, to avoid indeterminant forms. By factoring the second derivatives of the energy with respect to (α − π) and sinα, analysis in this local coordinate system avoids division by sinα as α → π. The formulas describe linear systems without the need to project the motion onto two perpendicular planes. When the angle is exactly π two degenerate directions arise, and the coordinate axes perpendicular to the axis of the linear sequence of atoms may be chosen arbitrarily. Further refinements and clarifications of methods reported previously to obtain matrix elements for linear and out‐of

ISSN:0192-8651    

DOI:10.1002/jcc.540110308

出版商:John Wiley&Sons, Inc.    

年代:1990

数据来源: WILEY

摘要:

AbstractAtomic partial charges for three model systems that mimic the metal‐ligand moiety of the active site in the enzyme Cu, Zn superoxide dismutase (SOD) have been calculated at theab initiolevel. The model systems include copper and zinc complexes with imidazole, formate and ammonia ligands. The partial charges thus obtained have been incorporated into force fields for molecular simulations. Simulations carried out with these force fields justify the need for specialized charge assignments for the metals and their ligand

ISSN:0192-8651    

DOI:10.1002/jcc.540110309

出版商:John Wiley&Sons, Inc.    

年代:1990

数据来源: WILEY

摘要:

AbstractΔH0, ΔS0, andKphave been calculated by force field methods for two prototype Diels–Alder reactions, the condensation of 1,3‐butadiene and ethylene, and the dimerization of 1,3‐butadiene. Three force field programs were employed, Allinger's MMP2, Warshel and Karplus' CFF/PI, and Boyd's MOLBD3 (with Anet and Yavari diene parameters). MMP2 and CFF/PI were combined to calculate reactant and product ΔH f0(by MMP2) and thermodynamic functionsS0,C P0, and (H T0−H O0) by use of CFF/PI molecular geometry and vibrational frequencies. MOLBD3 was used separately to calculate these properties. The combined MMP2, CFF/PI calculations exhibited excellent agreement with available experimental equilibrium data for both reactions, despite uncertainties in the number and energies of adduct conformers in the dimerization reaction. The MOLBD3 results were less satisfying, particularly for the dimerization reaction, because the entropy of 1,3‐butadiene was overestimated

ISSN:0192-8651    

DOI:10.1002/jcc.540110310

出版商:John Wiley&Sons, Inc.    

年代:1990

数据来源: WILEY

摘要:

AbstractAn improved method for computing potential‐derived charges is described which is based upon the CHELP program available from QCPE.1This approach (CHELPG) is shown to be considerably less dependent upon molecular orientation than the original CHELP program. In the second part of this work, the CHELPG point selection algorithm was used to analyze the changes in the potential‐derived charges in formamide during rotation about the CN bond. In order to achieve a level of rotational invariance less than 10% of the magnitude of the electronic effects studied, an equally‐spaced array of points 0.3 Å apart was required. Points found to be greater than 2.8 Å from any nucleus were eliminated, along with all points contained within the defined VDW distances from each of the atoms. The results are compared to those obtained by using CHELP. Even when large numbers of points (ca. 3000) were sampled using the CHELP selection routine, the results did not indicate a satisfactory level of rotatational invariance. On the basis of these results, the original CHELP program was found to be inadequate for analyzing internal

ISSN:0192-8651    

DOI:10.1002/jcc.540110311

出版商:John Wiley&Sons, Inc.    

年代:1990

数据来源: WILEY