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-frequency modes, including a progression in the methyl torsion. Fits of the torsional progression yield an S-1 barrier of 54 cm(-1), which is nearly a factor of 3 lower than the S-1 barrier of the related molecule, trans-p-methylstilbene. The lower barrier is attributed to a reduction in the asymmetry of the pi electron density at the two positions adjacent 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.