Photochemical equilibration/isomerization of p-, m-, and o-methylbenzonitrile

The phototransposition reactions in acetonitrile of p-, m-, and o-methylbenzonitrile have been studied. Any one of the three is converted to the other two by either a 1,2- or 1,3-isomerization in a primary photochemical step. However, the reactivities are quite different with the relative values for para:meta:ortho = 32:4:1. For both the para and meta isomers, extended irradiations approach a calculated steady-state composition of para:meta:ortho = 3:20:77. Quenching of the excited triplet state of the para and meta isomers with 2,4-dimethyl-1,3-butadiene indicates that these reactions are occurring from the excited Singlet state. Irradiation of selectively labeled 2,6-dideuterio-4-methylbenzonitrile demonstrates that only the cyano-substituted carbon undergoes migration.