Three Remarks on Molecular Orbital Theory of Complex Molecules

Abstract

Three suggestions are made and discussed concerning the generalized Hückel and related methods for treating the quantum chemistry of complex unsaturated molecules: (1) Justification for the assumptions of zero overlap and zero differential overlap resides in two facts: (a) For many molecules the atomic orbitals in the LCAO molecular orbitals may be replaced by the corresponding orthogonalized atomic orbitals of Löwdin, without effect on the molecular orbitals. (b) Integrals involving charge distributions which are products of orthogonalized atomic orbitals are small. (Observations previously made by several authors.) (2) Molecules isoelectronic with benzene are conveniently handled making use of molecular orbitals appropriate to the full benzene symmetry, for then deviations from benzene symmetry can be classified and treated systematically using symmetry combinations of basic integrals. (Formulas are given.) (3) The two‐center coulomb repulsion integrals, previously dealt with by rather arbitrary semiempirical procedures, may be expanded by multipole expansion methods, leaving the independent multipoles of each atomic orbital as its defining theoretical or semiempirical characteristics.