THE SPECTROSCOPIC AND PHOTOPHYSICAL EFFECTS OF THE POSITION OF METHYL SUBSTITUTION .1. 4-METHYLPYRIMIDINE AND 5-METHYLPYRIMIDINE

Laser-induced fluorescence excitation and dispersed fluorescence spectra of the first n-pi-* transition of jet-cooled 4- and 5-methylpyrimidine (4-mp and 5-mp) have been recorded and analyzed. In 5-mp, methyl substitution preserves many of the spectroscopic signatures of the unsubstituted pyrimidine molecule. Dispersed fluorescence spectra are used to assign most of the major features in the first 1000 cm-1 of the excitation spectrum. High resolution scans at the origin reveal a 0.20 cm-1 splitting of the origin arising from the 0a'1-0a'1 and 1e"-1e" internal rotor transitions. From this small splitting we deduce that the nearly free internal rotation of the methyl group in the ground state is carried over to the S1 state as well. In 4-methylpyrimidine, the reduction in symmetry accompanying methyl substitution (G12 to G6) results in allowed transitions to all in-plane fundamentals. The methyl group is seen to participate in the electronic transition to a greater degree in 4-mp than in 5-mp. We observe clear activity in both the C-CH3 stretch and C-CH3 in-plane bend in the dispersed fluorescence from the origin of 4-mp. 4-mp also differs remarkably from 5-mp in the magnitude of the barrier to methyl internal rotation in S0 and S1. By fitting the positions and intensities of internal rotor structure in ground and excited states we deduce a ground state barrier to internal rotation of V3" = 95 +/- 5 cm-1 and a best-fit excited state barrier of V3' = 745 cm-1, V6' = -100 cm-1. Ab initio calculations on 4-mp reproduce both the magnitude and shape of the experimental barrier to internal rotation in the ground state. The lowest energy methyl conformation places a hydrogen atom in the plane of the ring pointing away from the nitrogen lone pair. Finally, in both molecules we observe spectroscopic signatures of vibrational state mixing in the S1 state. Density-of-states calculations on both molecules using the experimentally determined internal rotor energy levels predict a similar density of same-symmetry states in the two molecules at a given energy. Experimental evidence is presented that the small V6 barrier in 5-mp leads to modest vibration/internal rotation coupling matrix elements of approximately 1 cm-1. The high V3 barrier in 4-mp is observed to give strong vibration/internal rotation coupling in the case of the 6a0(1) (0a1) level which shifts its position by 17 cm-1 from its companion 6a0(1) (1e) level due to interaction with an X1(3a2) vibration/internal rotation combination band.