Norrish Type I vs. Norrish-Yang Type II in the Solid State Photochemistry of CIS-2,6-DI(1-Cyclohexenyl)-Cyclohexanone: A Computational Study

A computational study of the reaction mechanism for the solid state photolysis of cis-2,6-di(1-cyclohexenyl)cyclohexanone ( 1 ) was carried out using density functional theory. Compound 1 was designed to explore the competition between several possible pathways, but mainly to discern from α-cleavage/decarbonylation (Norrish type I), and γ-hydrogen abstraction followed by cyclization (Norrish type II- Yang). The triplet state reaction coordinates for the type I and II photochemistry were obtained computationally. While calculations predict that the type I reaction should be predominant in the triplet state due to lower barrier heights for the process, the observed reactivity in the solid state favored the type II process. By correlating the results from calculations and experiments with singlet/triplet sensitizers/quenchers, it was found that the observed reactivity takes place along the singlet manifold.