Pi-Electron Valence Bond Theory; Application to Vinyl Chloride

A valence bond theory is developed which accounts for differential overlap up to second order in both the Hamiltonian and overlap matrices. Electron-density operators are defined, and used to compute electronic charge distributions, also up to second-order in differential overlap. The theory is applied to the pi electrons of vinyl chloride, in order to investigate the effects of (a) omitting some of the polar electronic arrangements from the set of basis functions, and (b) neglecting differential overlap. Of these two common approximations, the latter is found to be much more serious. When it is used, the theoretically estimated integral parameters lead to very poor charge distributions; they must be changed by factors of 2 or 3 in order to obtain reasonable results. The neglect of polar structures which contain double ionizations has only small effects, but it appears that in general all structures involving single ionizations must be included. The most general calculations made on vinyl chloride, including differential overlap and all polar valence bond functions, gives results which compare very favorably with the available molecular orbital calculations, as well as with experiment.