Dissociative Photoionization of Dimethylpyridines and Trimethylpyridine at 266 nm: Dynamics of Methyl Radical Release.

The 266 nm photolysis of various positional isomers of dimethylpyridines and trimethylpyridine was investigated by measuring the translational energy distribution of the methyl radical following {sp}C-C{sp} bond dissociation. The observed translational energy distribution is attributed to the dissociative photoionization in the cationic ground state following [1 + 1 + 1] three-photon absorption. The translational energy distribution profiles of the methyl radical were broad with the maximum translation energy in excess of 2 eV, which originates due to the dissociation of {sp}C-C{sp} bond ortho to the N atom in the ring. The dynamics of {sp}C-C{sp} bond dissociation in the cationic ground state of methylpyridines is marginally dependent on the number and position of the methyl groups; similar to xylenes, however, it is site-selective with the preferential cleavage of C-C bond in the ortho position to the pyridinic nitrogen atom, which is attributed to the relative stability of the resulting radical cation.