Internal Rotation of the MethylGroup in the ElectronicallyExcited State: o‐ andm‐Toluidine

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.