The spectroscopic and photophysical effects of the position of methyl substitution. II. 2-methylpyrimidine

Laser-induced fluorescence excitation and dispersed fluorescence spectra of the first n-?r* 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 4and 5methylpyrimidine. An unusual Fermi resonance involving the 6~2: progression forms the focus of the present study. The 6~2: 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 166 ’ (3~;‘) vibration/internal rotation combination level. This level is selectively coupled by symmetry constraints to 6a’ (0~; ), leaving the 6~2’ ( le” ) level unperturbed. The positions and intensities of the triad of peaks in the excitation spectrum allow a quantitative determination of the 6a’ (0~; )++16b ’ (3a;‘) coupling matrix element of V = 4.1 cm ‘. 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 6~2: 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 1 cm ‘.