Semiempirical Molecular Orbital Calculations. VII. σ-π Separability in the Mulliken-Wolfsberg-Helmholtz Approach

The most replete Mulliken-Wolfsberg-Helmholtz (M-W-H) MO calculations available to date2 suffer from a number of deficiencies. Of these, it is probable that the neglect of “atoms-in-molecules” effects is one of the more crucial. In other words, whereas the diagonal matrix elements Hii of the M-W-H energy matrix are readily evaluated from atomic ionization data in such a way as to take account of the charge and configuration of the particular atomic center on which the AO i is situate, they do not include any effects due to the neighboring atomic centers. These effects are probably complex-involving, as they must the full complement of electron repulsion and electron-nuclear penetration effects. A discussion of such effects in heteronuclear MXn molecules has been given in terms of a Madelung potential by Jorgenson et al. 3; these authors have shown that such neighbor effects dominate covalent bonding effects when the atoms M and X have significantly different electronegativies. A more replete discussion of this same topic has been given by Corrington and Cusachs4.