S1-S0 SUPERSONIC JET FLUORESCENCE-SPECTRA OF TRANS-2-METHYLSTYRENE - AN OUT-OF-PLANE VINYL GROUP IN S0

Single vibronic level fluorescence spectra of trans-2-methylstyrene in a supersonic jet show long progressions in the C(1)-C(alpha) 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-degrees. 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.