THEORETICAL STUDIES ON THE ELECTRONIC SPECTRA OF SUBSTITUTED AROMATIC MOLECULES: PART III. SEMIEMPIRICAL SCF CALCULATIONS FOR POLYCYCLIC ALTERNANT HYDROCARBONS

Abstract

The effect of changing the method of calculating repulsion integrals within the framework of the semiempirical self-consistent field method is investigated with respect to the transition energies of the alternant hydrocarbons benzene, anthracene, phenanthrene, and pyrene. Of the methods investigated, the one originally suggested by Mataga gave the best agreement with experiment. By statistical analysis of the linear relationship between the internuclear distance rpq and the bond order ppq, the regression line[Formula: see text]was obtained.The success of the Hückel LCAO procedure in correlating observed p-band transition energies with the Hückel energy number[Formula: see text](NP = No. of atoms contributing π electrons) for the highest occupied MO was shown to be due to (1) the relative unimportance of configuration interaction for this transition, (2) the invariance of the electron interaction terms calculated by the SCF method for different hydrocarbons, and (3) the identity of the Hückel energy number [Formula: see text] with the corresponding term[Formula: see text]of SCF theory, the latter value being insensitive to the method of choosing repulsion integrals even though the individual AO coefficients [Formula: see text] were quite sensitive.