No abstract

We describe an ab initio procedure for extracting the Pauli exchange antisymmetry (“steric”) contributions to molecular potential energy in the framework of self-consistent-field molecular orbital (SCFMO) theory. This “natural steric analysis” method is based on natural bond orbital (NBO) representation of the SCFMO wave function, which allows the steric exchange energy to be approximated as an energy difference between “preorthogonal” and final NBOs, analogous to the procedure of Sovers et al. [J. Chem. Phys. 49, 2592 (1965)]. We show how the total NBO steric exchange energy can in turn be approximated in terms of pairwise-additive interactions between localized bonding units, comparable to the empirical steric potentials of molecular mechanics models. The accuracy of NBO steric analysis is tested in applications to various rare-gas interactions (expected to be of pure steric exchange type), and excellent agreement is found with the full ab initio potential curves over a wide range of separations. Limitations of the simple pairwise-additive model of steric exchange interactions are noted and characterized.

ABSTRACT:We describe quantitative numerical applications of the natural Ž . resonance theory NRT to a variety of chemical bonding types, in order to demonstrate the generality and practicality of the method for a wide range of . chemical systems. Illustrative applications are presented for 1 benzene and . . polycyclic aromatics; 2 CO , formate, and related acyclic species; 3 ionic and 2 .. polar compounds; 4 coordinate covalent compounds and complexes; 5 . hypervalent and electron-deficient species; 6 noncovalent H-bonded complex;. and 7 a model Diels-Alder chemical reaction surface. The examples exhibit the general harmony of NRT weightings with qualitative resonance-theoretic concepts and illustrate how these concepts can be extended to many new types of chemical phenomena at a quanitative ab initio level. ᮊ

We employ natural steric analysis (introduced in a previous paper) to evaluate a set of effective ab initio van der Waals radii for free and covalently bonded atoms and ions of H–Ar (Z=1–18) determined using a helium atom probe. We critically examine the degree of anisotropy, dependence on charge state, and other intrinsic limitations of a simple atomic van der Waals hard sphere representation of the accurate steric surface. We also evaluate the ab initio steric force (gradient of steric energy at van der Waals contact) as a measure of “hardness” of the atomic van der Waals spheres. Comparison with empirical van der Waals radii shows reasonable agreement (within the acknowledged uncertainties of the latter values in the most important cases), but suggest a wider range of variability and anisotropy than could be adequately represented by any fixed constant radius. Simple expressions for incorporating the dependence on natural atomic charge or correcting for other types of intermolecular contact are given, extending the accuracy and usefulness of the atomic van der Waals sphere concept.

ABSTRACT:We apply natural bond orbital NBO steric analysis introduced in a . previous article to obtain the steric exchange contribution to the internal rotation barriers Ž . of butane, ethane, and other related molecules CH NH , CH OH, NH OH . The expected exchange repulsion between the two methyl group C-H bonds within van der Waals contact in butane is shown to be the major contributor to the syn barrier and provides a method for calculating allowed ranges of torsional angles in macromolecules. However, the exchange-energy difference between the staggered and eclipsed forms predict a counterintuitive eclipsed minimum for ethane, methylamine, and methanol. We show that the full SCF barrier in such apolar molecules can be reasonably approximated as a sum of this steric term plus the hyperconjugative terms as previously evaluated.