THE SPECTROSCOPIC AND PHOTOPHYSICAL EFFECTS OF THE POSITION OF METHYL SUBSTITUTION .2. 2-METHYLPYRIMIDINE

Laser-induced fluorescence excitation and dispersed fluorescence spectra of the first n-pi* transition of jet-cooled 2-methylpyrimidine have been recorded and analyzed. This work extends our earlier study of the spectroscopic and photophysical effects of methyl substitution in 4- and 5-methylpyrimidine. An unusual Fermi resonance involving the 6a0n progression forms the focus of the present study. The 6a(o)1 vibronic transition is observed to be split into a triad of transitions. Dispersed fluorescence spectra are used to identify the dark background state responsible for the Fermi resonance coupling as the 16b 1(3a'2') vibration/internal rotation combination level. This level is selectively coupled by symmetry constraints to 6a1(Oa'1), leaving the 6a1(1e") level unperturbed. The positions and intensities of the triad of peaks in the excitation spectrum allow a quantitative determination of the 6a1(Oa'1) <--> 16b 1(3a2") coupling matrix element of V = 4.1 cm-1. This vibration/internal rotation Fermi resonance is thus typical of the new types of routes to vibrational state mixing which are opened by methyl substitution. Higher members of the 6a0n progression are also involved in Fermi resonance mixing. However, in addition, these levels experience weaker, less state-specific coupling to a bath of same-symmetry states at that energy. The excitation spectrum provides an estimate of the average coupling matrix element of this second tier coupling of approximately 1 cm-1.