Molecular Fragments of Heterocyclic Aromatic Compounds. II

Abstract

Pi-electron approximations, and semiempirical theories based on them, have come to be widely used, not only for pi-electron (planar-conjugated) systems alone but for other molecules as well. Inherent in all of these methods is the LCAO-MO approach. Thus, when one gets down to the “nitty gritty,” integrals over AO's become the basic units which need somehow to be estimated. Even in those cases of gross empiricism, overlap integrals appear and cannot be estimated empirically. Thus the AO's used need to be specified. In this work an attempt has been made to determine AO's to be used in defining MO's, which are conditioned to a molecular environment designed to simulate fragments of heterocyclic aromatic compounds and thus to go beyond the use of AO's determined for free atoms. In this spirit, numerous planar hydrides of B, C, N, and O have been handled within a one-center representation of an open- and closed-shell Hartree-Fock approach and the results presented.