A CNDO/S study on the electronic structure of some methyl and phenyl substituted pyryliums

Abstract

The semiempirical molecular orbital CNDO/S spectral parameterization of Del Bene and Jaffe has been used to elucidate the electronic structure of a series of methyl and phenyl substituted pyrylium cations. This study demonstrates that the CNDO/S technique, without additional parameterization, provides a quantitative basis for the understanding of various ground and excited singlet state properties of the five pyrylium molecules considered. Of particular importance is the favorable comparison of experimental and calculated relative oscillator strengths and the magnitude and direction of shifts of the singlet transitions. The nature of the low-lying singlet states are further elucidated through a study of the calculated wavefunctions. Comparisons of potential energy diagrams for the interaction of the ground and excited state static charge density distributions with a negative test charge demonstrates that the nature of the transition could be important in the determination of relative spectral sensitization properties.