INTERNAL ROTATION, LOW FREQUENCY VIBRATIONS AND THEIR INTERACTIONS IN p-FLUOROTOLUENE AND ITS -$d_{3}$ DERIVATIVE

$^{1.}$ C.S. Parmenter and B.M. Stone, J. Chem. Phys., 84, 4710 (1986). $^{2.}$ D.B. Moss, C.S. Parmenter and G.E. Ewing, J. Chem. Phys., 86. 51 (1987). $^{3.}$ K. Okuyama, N. Mikami, and M. Ito, J. Phys. Chem., 89, 5617 (1985).Author Institution: Department of Chemistry, Indiana UniversityIn a continuing study of interactions between methyl group internal rotation and ring $vibrations^{1,2}$ we have revisited the $S_{1}-S_{0}$ fluorescence spectroscopy. Improved resolution has yielded the rotational contours of low energy bands in the excitation spectra as well as many new features in the dispersed fluorescence spectra. The $-d_{3}$ spectra allow secure distinction between bands involving ring vibrations and those involving methyl group internal rotation. Franck-Condon intensities are abnormal. More than four types of rotational contours occur in the excitation bands. Several assignments by Ito $et al.^{3}$ in the excitation spectrum have been revised and additional assignments have been made. Franck-Condon activities and band contours are explained by interactions among electronic motion, vibration, internal rotation and overall rotation. Abnormal Franck-Condon activities in the dispersed fluorescence spectra tell us which internal rotation states couple with which vibrational states and whether this coupling is turned on by overall rotation and nuclear spin