摘要:

AbstractThe standard MM2 force field and program have been modified to include SiO groups, the force constants being derived fromab initiowavefunctions, and using partial atomic charges calculated from the empirical algorithm CHARGE2. Molecular mechanics calculations have been carried out on disiloxane, dimethoxy‐dimethylsilane, methyl silyl ether, and cyclotrisiloxane, and reasonable agreement has been obtained between the observed and calculated geometries. In addition, the energies obtained using the modified force field have been tested against experimental data on alkyl silanes where energetic and structural information is available. Again good agreement is obtained. The application of the force field to extended SiO systems will also be disc

ISSN:0192-8651    

DOI:10.1002/jcc.540090702

出版商:John Wiley&Sons, Inc.    

年代:1988

数据来源: WILEY

摘要:

AbstractModel potential parameters and basis sets, presented previously for the transition metal atoms Sc through Hg, are tested in calculations of the transition metal compounds (CuF, CuCl, Cu2, TiCl4, ZrCl4, CoF63−, CoF62−, AgH, AuH, CrF6, ScO, ZrO, Cr2, Mo2). Calculated values of the bond distances, vibrational frequencies, and some transition energies (for Cu2and CoF62−) are compared with those given by all‐electron calculations with basis sets of high quality. Singlet‐triplet splittings in Cu2and correlation energies in CrF6n−(n = 0, 1, and 2) are also examined. The satisfactory results obtained by these calculations strongly support the contention that the model potential method is a reliable and economical alternative to theab initioHartree‐Fock‐

ISSN:0192-8651    

DOI:10.1002/jcc.540090703

出版商:John Wiley&Sons, Inc.    

年代:1988

数据来源: WILEY

摘要:

AbstractThe Hammond postulate is a useful, qualitative tool that interrelates structural similarities between reactants, transition structures, and products with the exo‐ or endothermicity of reactions. It applies to most chemical reactions, although several exceptions are known. In this study the following problem is addressed: is it possible to formulate conditions for the validity of the quantitative Hammond postulate in terms of simple physical quantities characteristic to the molecules involved? A detailed analysis is given for the conditions of validity of the postulate, in terms of bounds on theinternal forcesandforce constantsof nuclear arrangements encountered along a reaction path. We have determined a broad class of constraints on barrier shapes that must be satisfied in order to obtain a critical situation thatviolatesthe Hammond postulate: a reactant‐like transition structure (“transition state”) for endothermic reactions, and a product‐like one for exothermic reactions. The general constraints are formulated in terms of physically meaningful quantities: (i) energy differences, (ii) restrictions on slopes (e.g., an upper bound on internal forces), and (iii) restrictions on curvatures (e.g., upper bounds on force constants) along potenti

ISSN:0192-8651    

DOI:10.1002/jcc.540090704

出版商:John Wiley&Sons, Inc.    

年代:1988

数据来源: WILEY

摘要:

AbstractAb initiowave functions using the 6‐31G** basis set were calculated for a set of 24 organic molecules which included saturated and unsaturated hydrocarbons, fluoro compounds, hydroxyl and ether compounds, carboxylic acids, esters, amines, carbonyl compounds, amides, andN‐methyl amides. The electric potential on a grid within a van der Waals shell around each molecule was calculated directly from the wavefunctions. The electric potential values were modeled by placing multipoles up to quadrupoles at atomic sites. The electric potential was well fitted by a model which included atomic monopoles, dipoles, and quadrupoles. Also, the electric potential was modeled by placing dipoles at bond centers. This bond dipole was either allowed to point in any direction, or was restricted to the bond direction. In general the values shown for the monopoles and the values and directions for the bond dipoles were as expected from considerations of electronegativity and reactivity. For the general bond dipole model in a number of cases the direction of the bond dipole was not parallel to the bond direction. It is suggested that this is an artifact caused by the effects of lone pair electrons or electron delocalization on the model. The restricted bond dipole model fitted the electric potential about as well as the atomic monopole model. Atomic monopole values (net atomic charges) and bond dipole values for various atoms, functional groups, and bonds are discussed. Since bond‐dipole interaction energy has better long‐range convergence than monopole interaction energy, bond dipoles are a useful alternative to atomic mo

ISSN:0192-8651    

DOI:10.1002/jcc.540090705

出版商:John Wiley&Sons, Inc.    

年代:1988

数据来源: WILEY

摘要:

AbstractModifications are made to a previously developed scheme for calculating atomic charge which uses orbital electronegativity and which requires minimal calculational effort. The introduced changes are a result of deficiencies noted in the earlier method which were due to an inadequate accounting of effects from neighboring atom charges. Results obtained using the modified scheme for both model compounds as well as larger molecules of interest to biochemistry are compared to previous results and also to several levels ofab initiocalculations. It is shown that a definite improvement is obtained and that the present method gives very good correlations with each calculational level. Comparisons are also made with other methods that use electronegativity theory. It is shown that the present scheme represents a definite improvement over alternate orbital electronegativity methods and is roughly comparable to a higher level scheme that utilizes atomic electronegativity values. A discussion comparing the latter method with the present one is included. Because of the small amount of calculational effort involved, the results indicate that the present method could be quite useful in providing reliable atomic charges for large molecular systems.

ISSN:0192-8651    

DOI:10.1002/jcc.540090706

出版商:John Wiley&Sons, Inc.    

年代:1988

数据来源: WILEY

摘要:

AbstractMethods for the computation of one‐ and two‐electron spin‐orbit integrals over Gaussian‐type basis functions are presented. We show that existing nuclear‐attraction and electron‐repulsion integral codes can be readily adapted for the efficient evaluation of spin‐orbit integrals; in particular, one can take advantage of recent advances in the computation of derivative integrals. Recurrence relations for the nuclear attraction integrals are a

ISSN:0192-8651    

DOI:10.1002/jcc.540090707

出版商:John Wiley&Sons, Inc.    

年代:1988

数据来源: WILEY

摘要:

AbstractA number of important reactions involved in the formation and metabolism ofN‐nitrosamines have been studied using a modified reaction field method which treats the solute quantum mechanically and the solvent as a polarized dielectri

ISSN:0192-8651    

DOI:10.1002/jcc.540090708

出版商:John Wiley&Sons, Inc.    

年代:1988

数据来源: WILEY

摘要:

AbstractTheoretical calculations at the 3‐21G and 3‐21 + Gab initiolevels and at the MNDO and AM1 semiempirical levels of several six‐membered nitrogenated heterocycles and their protonated species have been carried out. The 3–21G calculated proton affinities are systematically too high, in relation to the available experimental data, and it is estimated that inclusion of electron correlation and zero‐point corrections is not sufficient to reach the desired agreement; however, additional inclusion of diffuse functions (3‐21 + G/3‐21G calculations) lowers the calculated proton affinities by 5.4–6.8 kcal/mol, a good agreement being thus obtained, at least for1–7. On the other hand, semiempirical methods underestimate the repulsion between each pair of vicinal nitrogens; however, if a correction of ˜9 kcal/mol is added to the AM1 results for each pair of neighboring nitrogens containing lone pairs of electrons, the corresponding proton affinities match fairly well the available exoerimental data and corrected 3‐21 + G results. As expected, all methods predict that the introduction of additional nitrogens decreases the overall absolute basicity. Futhermore, comparison of the relative basicity of the isomers and of the preferred protonation site for each isomer indicates that nitrogen atoms with (only) one α‐nitrogen and without a γ‐nitrogen are more basic than any others. In benzazines,MNDOandAM1suggest that the 2,3‐diaza arrangement has a higher intrinsic basicity th

ISSN:0192-8651    

DOI:10.1002/jcc.540090709

出版商:John Wiley&Sons, Inc.    

年代:1988

数据来源: WILEY

摘要:

AbstractWe present a new algorithm for the calculation of two‐center one‐electron integrals withSTOSbased on two sets of recurrence relations. The first one enables us to calculate any of these integrals in terms of a few “basic integrals.” Furthermore, these basic integrals are written in terms of certain auxiliary functions which can be obtained through the second set of relations from only two starting values. We give also simple formulas for these starting quantities. Finally, the numerical stability, accuracy, and speed of the different steps of the algorithm are a

ISSN:0192-8651    

DOI:10.1002/jcc.540090710

出版商:John Wiley&Sons, Inc.    

年代:1988

数据来源: WILEY

ISSN:0192-8651    

DOI:10.1002/jcc.540090711

出版商:John Wiley&Sons, Inc.    

年代:1988

数据来源: WILEY