Rotational isomerism in trans-1,2-diarylethylenes. A Raman study of conformational equilibrium of styrylnaphthalenes

Abstract

Rotational isomerism in styrylnaphthalenes has been investigated by Raman vibrational spectroscopy. Evidence was found for the existence of two rotational, s-trans and s-cis, isomers in solutions of trans-2-styrylnaphthalene, and by analysis of the temperature dependence of relative band intensities an enthalpy difference of 0.71 ± 0.07 kcal mol^–1 was derived for the equilibration reaction. Only one form was found for trans-1-styrylnaphthalene, where one of the two corresponding isomers is hindered by strong steric interactions. The results of this work greatly increase the evidence for conformational equilibria in trans-1,2-diarylethylenes previously obtained by fluorescence studies as well as predicted by molecular-orbital calculations.