Theoretical Study of Optical Absorption Curves of Molecules. IV: Improvement of Self-Consistent HMO Method Using a New Resonance Integral and Investigation of a Relation between Bond Alternation and Wavelength of Optical Absorption

A self-consistent HMO theory proposed previously is improved by adding a correction term, which emerges from a correlation between bond orders, to a resonance integral. Although the correction term is so small and is neglected in the theories of Pariser, Parr and Pople, its role in an enhancement of bond alternation in polyenes is very large due to its cooperative effect. By this, absorption wavelengths of many conjugated molecules are rightly calculated. Our theory is also applied to heteroatoms-conjugated molecules and molecules with distorted bonds. Self-consistently determined bond lengths of various molecules agree with experimental values within an error of 0.03Å.