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 So 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.