Stereoisomerism and spectroscopic properties of α,ω-diphenylpolyenes. Part 1.—Theoretical study on the shape of the long-wavelength u.-v. band

Abstract

As a first step in the interpretation of the spectroscopic changes connected with stereoisomerization of α,ω-diphenylpolyenes, a “quantitative” justification for the diffuse spectrum of sterically-hindered isomers in the near u.-v. region has been attempted taking trans- and cis-stilbene as test molecules. Potential energy curves have been obtained for two low-frequency vibrational modes, namely the twisting of the essential single bonds and the torsion of the central double-bond, in both the initial (¹A) and final (¹B) electronic states. The corresponding sets of vibrational energies and wavefunctions have been obtained by expressing the potential functions as power series and by expanding the wavefunctions in a basis of harmonic oscillator eigenstates. Franck-Condon distributions calculated for the the two vibrational modes have shown that the “twisting” rather than the “torsion” mode is responsible for the blurring of the fine structure in the conjugation band of cis-stilbene.