On the Role of Spin Inversion in the Triplet Photochemistry of Benzyl Halides and Benzylammonium Salts

The direct photolysis of benzyl halides and benzylammonium salts yields photoproducts from both diradical and carbonium ion intermediates. The sensitized photolysis yields similar results in the case of the benzyl halides but gives only products derived from a diradical intermediate in the case of the ammonium salt. This selectivity is analyzed qualitatively by using a valence bond approach coupled with ab initio MO-CI calculations. It is shown that the lowest triplet state of a benzyl halide at a point prior to dissociation is the ³(n_x → σ*_cx); in contrast, the ³(σC_N → ς*_CN) state is found to be the lowest triplet of the ammonium salt. The former is shown to have an efficient spin-inversion mechanism leading to an ion pair while the latter does not.