C(1)-C(ALPHA) TORSION POTENTIAL FUNCTION AND VIBRATIONAL ASSIGNMENTS OF TRANS-BETA-METHYLSTYRENE FROM S-1-S-0 SUPERSONIC JET FLUORESCENCE-SPECTRA

The fluorescence excitation spectrum and the single vibronic level dispersed fluorescence spectra for the ($) over tilde A(1)A' <-- ($) over tilde X(1)A' transition of trans-beta-methylstyrene are reported. Together with information from the high-resolution infrared spectrum, vibrational assignments are made in the ground and excited states. A short vinyl torsion vibrational progression has been observed in S-0 and fitted to a V(phi) = Sigma(n)V(n)(1 - cos n phi) potential with V-2 = 855 cm(-1) and V-4 = -218 cm(-1) which, Like that for styrene, is flat-bottomed. A possible assignment for a transition involving the methyl torsional motion is made, and the results are compared with the analogous motion in propene. The role of intramolecular vibrational redistribution is also assessed with relevance to intermode coupling of the system modes to low-frequency modes of the substituent group in the excited state.