SEMIEMPIRICAL MOLECULAR ORBITAL THEORY AND MOLECULAR GEOMETRY .I. ANALYTIC PROCEDURES FOR EXTENDED HUCKEL METHODS

Analytic expressions for orbital electronic energies and charge distributions for the first-row atom dihydrides are developed within the framework of two important variations of extended-Hückel theory. The major purpose of the development is to ascertain in detail the individual effects operative within the different molecular symmetries and how these vary with symmetric changes in geometry. Fairly simple expressions are derived for angular changes both within the original and the kinetic-energy-included methods, and the balancing of energy effects between the a1 and b2 symmetry orbitals appear therefrom to be capable in some cases of qualitatively reproducing the correct equilibrium molecular configurations. Correspondences with the Walsh rules are explored. Reasons for agreement with these rules and with empirical structures when found are discussed; why the agreement can be no more than qualitative is also considered. The more complicated expressions for symmetric bond stretching and contraction in these molecules are similarly derived and examined, and some reasons for the general failure of the methods to represent these deformations are detailed. © 1969, American Chemical Society. All rights reserved.