REMOTE SUBSTITUENT EFFECTS ON METHYL TORSIONAL BARRIERS - TRANS-P-AMINO-P'-METHYLSTILBENE

The expansion-cooled fluorescence excitation spectrum of trans-p-amino -p'-methylstilbene is presented. It displays several prominent low-fre quency modes, including a progression in the methyl torsion. Fits of t he torsional progression yield an S-1 barrier of 54 cm(-1), which is n early a factor of 3 lower than the S-1 barrier of the related molecule , trans-p-methylstilbene. The lower barrier is attributed to a reducti on in the asymmetry of the pi electron density at the two positions ad jacent to the CH3 caused by the strongly donating amino group. This is facilitated by delocalization of the pi system in the excited state. Other data, such as the behavior of the low-frequency ring motions and the crystal structure, are presented in support of this model.