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

摘要

Laserhyphen;induced fluorescence excitation and dispersed fluorescence spectra of the firstnndash;pgr;ast; transition of jethyphen;cooled 4hyphen; and 5hyphen;methylpyrimidine (4hyphen;mp and 5hyphen;mp) have been recorded and analyzed. In 5hyphen;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 cmminus;1of the excitation spectrum. High resolution scans at the origin reveal a 0.20 cmminus;1splitting of the origin arising from the 0arsquo;1ndash;0a1and 1elsquo;ndash;1elsquo;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 theS1state as well. In 4hyphen;methylpyrimidine, the reduction in symmetry accompanying methyl substitution (G12toG6) results in allowed transitions to all inhyphen;plane fundamentals. The methyl group is seen to participate in the electronic transition to a greater degree in 4hyphen;mp than in 5hyphen;mp.We observe clear activity in both the Cndash;CH3stretch and Cndash;CH3inhyphen;plane bend in the dispersed fluorescence from the origin of 4hyphen;mp. 4hyphen;mp also differs remarkably from 5hyphen;mp in the magnitude of the barrier to methyl internal rotation inS0andS1. By fitting the positions and intensities of internal rotor structure in ground and excited states we deduce a ground state barrier to internal rotation ofVlsquo;3=95plusmn;5 cmminus;1and a besthyphen;fit excited state barrier ofV3=745 cmminus;1,Vrsquo;6=minus;100 cmminus;1.Abinitiocalculations on 4hyphen;mp which 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 theS1state. Densityhyphen;ofhyphen;states calculations on both molecules using the experimentally determined internal rotor energy levels predict a similar density of samehyphen;symmetry states in the two molecules at a given energy. Experimental evidence is presented that the smallV6barrier in 5hyphen;mp leads to modest vibration/internal rotation coupling matrix elements of sim;1 cmminus;1. The highV3barrier in 4hyphen;mp is observed to give strong vibration/internal rotation coupling in the case of the 6a10(0a1) level which shifts its position by 17 cmminus;1from its companion 6a10(1e) level due to interaction with anX1(3a2) vibration/internal rotation combination band.