Methyl and vinyl torsional potentials in cis- and trans-3-methylstyrene from supersonic jet fluorescence spectra

Abstract

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 S₀ and involving just the methyl torsion in S₁. The resulting torsional levels have been fitted to a ½Σ_nV_n(1 – cos nϕ) potential with V^″₁= 8.9 cm^–1, V^″₂= 1212 cm^–1 and V^″₄=–302 cm^–1 for the vinyl torsion in S₀. 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 S₀, 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 S₀, V^′₃= 79 cm^–1 and V^′₆= 5 cm^–1 in S₁ of the trans rotamer.