METHYL AND VINYL TORSIONAL POTENTIALS IN CIS-3-METHYLSTYRENE AND TRANS-3-METHYLSTYRENE FROM SUPERSONIC JET FLUORESCENCE-SPECTRA

Part of the fluorescence excitation spectrum and several single vibronic level dispersed fluorescence spectra of the cis and trans rotamers of 3-methylstyrene in a supersonic jet have been obtained. Both rotamers show overlapping progressions involving methyl and vinyl torsional vibrations in S(o) and involving just the methyl torsion in S1. The resulting torsional levels have been fitted to a 1/2 SIGMA-n V(n)(1 - cos n-phi) potential with V"1 = 8.9 cm-1, V"2 = 1212 cm-1 and V"4 = -302 cm-1 for the vinyl torsion in S(o). Both rotamers are very similar in energy with the cis rotamer probably being the more stable, and, although they are planar, both of them show, like styrene, a very flat-bottomed potential. The methyl torsional levels have been fitted with V"3 = 45 cm-1 in S(o), V"3 = 165 cm-1 and V'6 = - 13 cm-1 in S1 of the cis rotamer, and V"3 = 52 cm-1 and V"6 = 19 cm-1 in S(o), V'3 = 79 cm-1 and V'6 = 5 cm-1 in S1 of the trans rotamer.