Theoretical studies of internal methyl rotations in m-xylene: comparison of Franck-Condon factors with the experimental spectra

摘要

The potential energy surfaces of internal rotations of two methyl groups were calculated with the ab initio MO method for m-xylene (1, 3-dimethylbenzene) in the ground (S_0), first excited (S_1), and cation ground (D_0) states. The internal rotation levels and their wavefunctions on the theoretically calculated potential energy surfaces were obtained by solving Schrodinger's equation for double methyl rotors. The Franck-Condon factors for D_0 <- S_1 and S_1 <- S_0 transitions were calculated and compared with the experimental spectra. A good agreement was obtained not only in level spacings but also in relative intensities in the spectra. The interaction between two methyl rotors was also found to be small, and the rotational levels can be labeled by the irreducible representation of the direct product G_6 * G_6 group.