Internal Rotation of the Methyl Group in the Electronically Excited State: o- and m-Toluidine

Abstract

The fluorescence excitation and dispersed fluorescence spectra of jet-cooled o- and m-toluidine were observed. Vibrational analysis of the spectra provided us with the potentials for the internal rotation of the CH₃ group in both ground and excited states. In o-toluidine, a large potential barrier to the internal rotation in the ground state is practically removed in the excited state. On the other hand, a nearly free internal rotation of the CH₃ group in the ground state of m-toluidine gains a large barrier by the electronic excitation. The great change in the barrier height upon the electronic excitation is more remarkable than that found for fluorotoluene. A close relationship between the barrier height and the π electron density at the ring carbon atom was found, indicating the hyperconjugation as the origin of the barrier height in the absence of steric hindrance.