The spectroscopic and photophysical effects of the position of methyl substitution. I. 4‐ and 5‐methylpyrimidine

Laser‐induced fluorescence excitation and dispersed fluorescence spectra of the first n–π* 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 6an0 progression forms the focus of the present study. The 6a10 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 16b1(3a’’2) vibration/internal rotation combination level. This level is selectively coupled by symmetry constraints to 6a1(0a1), 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(0a’1)↔16b1(3a2) coupling matrix element of V=4.1 cm−1. This vibration/internal rotation Fermi resonance is thus t...