S1–So supersonic jet fluorescence spectra of trans-2-methylstyrene : an out-of-plane vinyl group in So

Abstract

Single vibronic level fluorescence spectra of trans-2-methylstyrene in a supersonic jet show long progressions in the C(1)—C(α) torsional vibration of the vinyl group relative to the benzene ring. The first 11 torsional vibrational levels in S_o have been fitted to a potential function with a barrier of 151 cm^–1 to coplanarity of the vinyl group and the benzene ring. The equilibrium out-of-plane angle is 33°. In S₁ all the evidence from the fluorescence excitation spectrum points to a coplanar, or nearly coplanar, vinyl group. Identification of transitions involving methyl torsional motion is doubtful but might indicate a high torsional energy barrier in both electronic states. The cis rotamer has not been found.