Journal of Computational Chemistry

doi: 10.1002/jcc.540100101pmid: N/A

Liu, Xiaoyu; Balasubramanian, K.

doi: 10.1002/jcc.540100102pmid: N/A

A computer code and nonnumerical algorithm are developed to construct the edge group of a graph and to enumerate the edge colorings of graphs of chemical interest. The edge colorings of graphs have many applications in nuclear magnetic resonance (NMR), multiple quantum NMR, enumeration of structural isomers of unsaturated organic compounds, and in the construction of configurational integral expansion series in statistical mechanics. The code developed is applied to many NMR graphs, complete graphs containing up to 10 vertices, and the Petersen graph.

Gimarc, Benjamin M.; Dai, Baiching; Ott, Jane J.

doi: 10.1002/jcc.540100103pmid: N/A

We report the geometry‐optimized total energies and bond distances for the closo‐carborane isomers 3,5‐C2B6H8, 1,7‐C2B7H9, and 1,2‐C2B7H9 calculated by the ab initio SCF MO method using the STO‐3G basis set. Relative energies are compared with those of the other carborane isomers in the 8‐ and 9‐ atom classes. These results complete the set of calculations at the same level of theory for all deltahedral carborane isomers except for those of the 11‐atom class.

Chung‐Phillips, Alice

doi: 10.1002/jcc.540100104pmid: N/A

The procedure for deorthogonalization (D) of atomic orbitals in the semiempirical CNDO approach is reviewed. For comparative studies, CNDO/2, CNDO/2D, and STO‐3G calculations of molecular dipole moments and Mulliken populations are carried out on 35 prototype molecules containing H, C, N, O, and F atoms. The calculated values are assessed on the basis of how well they agree with experimental trends, chemical bonding theories, and ab initio molecular orbital (MO) values. Results of analyses indicate that the CNDO/2D values for dipole moments are in reasonable agreement with experimental values, and those for net atomic charges and electron populations bear greater resemblance to the ab initio (STO‐3G and 6‐31G**) values than the original CNDO/2 values. These findings, together with those of previous investigators, demonstrate unequivocally the advantages of incorporating deorthogonalization into routine CNDO/2 or INDO calculations as a means to obtain reasonable estimates of charge distributions.

Williams, Martin L.; Gready, Jill E.

doi: 10.1002/jcc.540100105pmid: N/A

An unusual type of π‐electron delocalization in Y‐shaped molecules related to guanidine and its protonated form, the guanidinium ion, has been studied by ab initio methods at the STO‐3G and 3‐21G levels. Results are reported for tautomeric, rotameric, and protonated forms of the oxygen‐substituted guanidine series (urea, carbamic, and carbonic acids); “extended‐guanidine” (aminomethylene guanidine) including pseudocyclic forms; and simple ring systems in which the extended‐guanidine group is incorporated (3‐amino‐1,2,4‐triazole, 2,4‐diaminopyrimidine). Both the guanidine and guanidinium type stabilizations have been characterized in terms of a number of structural and energetic parameters: degree of single/double bond character from bond lengths and π‐bond orders, electron distributions, and protonation energies. The major finding is that the structural and energetic properties of the isolated extended‐guanidinium group resemble those of the group when incorporated within 6‐membered heterocyclic or heterobicyclic rings, although the details vary with the nature of the ring and possibility of reinforcement or interference with the substructure resonance from overall ring delocalization. The implications for stabilization of the protonated forms of some biologically important pteridines is discussed.

Lopez, Jesus P.

doi: 10.1002/jcc.540100106pmid: N/A

The determination of minima and saddle points on the potential energy surfaces of the hydrogen bonded species O2−HF and O2−H2O is performed with unrestricted Hartree‐Fock calculations. Geometries, electron density distributions, and relative energies for every stationary point are reported. Only one true minimum is found for O2−HF and for O2−H2O, and this approximately corresponds to a structure where the partially positive hydrogen atom is located along one of the superoxide ion electron lone‐pair directions. Calculated ΔH, ΔS, and ΔG values for the reaction between O2− and H2O are in good agreement with experimental data.

Miller, Kenneth J.; Hinde, Robert J.; Anderson, Janet

doi: 10.1002/jcc.540100107pmid: N/A

Matrix elements for the first and second derivatives of the internal coordinates with respect to Cartesian coordinates are reported for stretching, linear, nonlinear, and out‐of‐plane bending and torsional motion. Derivatives of the energy with respect to the Cartesian coordinates are calculated with the chain rule. Derivatives of the energy with respect to the internal coordinates are straightforward, but the calculation of the derivatives of the internal coordinates with respect to the Cartesian coordinates can be simplified by the following two steps outlined in this article. First, the number of terms in the analytical functions can be reduced or will vanish when the derivatives of the bond length, bond angle, and torsion angle are reported in a local coordinate system in which one bond lies on an axis and an adjacent bond lies in the plane of two axes or is projected onto perpendicular planes for linear and out‐of‐plane bending motion. Second, a simple rotation transforms these derivatives to the appropriate orientation in the space‐fixed molecular coordinate system. Functions of the internal coordinates are invariant with respect to translation and rotation. The translational invariance and the symmetry of the second derivatives for a system with L atoms are used to select L‐1‐ and L(L‐1)/2‐independent first and second derivatives, respectively, of which approximately half of the latter vanish in the local coordinate system. The rotational invariance permits the transformation of the simplified derivatives in the local coordinate system to any orientation in space. The approach outlined in this article simplifies the formulas by expressing them in a local coordinate system, identifies the most convenient independent elements to compute, from which the dependent ones are calculated, and defines a transformation to the space‐fixed molecular coordinate system.

Ehrenson, S.

doi: 10.1002/jcc.540100108pmid: N/A

Radial dielectric constant (permittivity) functions for ionic solute, polar solvent systems of the type obtainable from the Lorentz‐Debye continuum field formulations are reexamined. Major interest is focused on the assumptions underlying these formulations and their expression in limiting field behavior. The analysis is extended to dipolar solutes and the importance of two types of corrections are evaluated. The first draws connections with the concept of the reaction field as employed by Onsager. This correction is shown to be significant as regards range of predicted saturation effects and for dipole moment self‐consistency, for the same type molecule serving as solute and solvent. The second type correction involves the phenomenon of electrostriction whose effects appear much more limited both in range and on the intensity of the fields necessary for its observation. Application of the permittivity functions developed to compute modified Born model hydration energies for a variety of ions is illustrated. Excellent asymptotic approximations for all radial permittivity equations of interest are also presented which should enhance their future utility.

Sotomatsu, Tomoko; Murata, Yoshiyuki; Fujita, Toshio

doi: 10.1002/jcc.540100109pmid: N/A

Theoretical analysis of the electronic effect of aromatic substituents was done with the use of the AM1 computational procedure. The gas‐phase acidity of substituted benzoic acids was linear with the difference in the heat of formation between corresponding benzoic acids and benzoate anions, the energy of the highest occupied molecular orbital, and the net charge on the acidic oxygen atoms of the corresponding benzoate anions. The Hammett σ constant was linearly correlated with the net charge on the atoms of the acid moiety of substituted benzoic acids. The AM1 computational procedure satisfactorily reproduced the electronic properties of a wide variety of substituents.

Profeta, Salvatore; Unwalla, Rayomand J.; Cartledge, Frank K.

doi: 10.1002/jcc.540100110pmid: N/A

For a series of simple alkyldisilanes, 3‐21G (*) full gradient geometry optimizations have been performed to yield both structural and conformational energy data which was suitable for calibrating the MM2 force field for disilanes. We have examined several model structures which yielded sufficient information about the rotational potential around the Si–Si bond to enable us to revise and augment those reported by Frierson. These parameters were questioned by us in the course of MM2 studies of 1,2‐disilacyclobutanes. We report new Si–Si torsion parameters as well as pertinent structural data from 3‐21G(*) geometry optimizations and relative conformational energies derived from Møller‐Plesset (MP2/MP3) calculations at the 6‐31(*)/3‐21G(*) level. The new parameters were applied to the 1,2‐dimethyl‐1,2‐disilacyclohexane system and those results are also reported.