Adjacent keto and enol groups in photochemistry of a cyclic molecule: Products, mechanisms and dynamics

The photochemistry of carbonyl compounds is of considerable atmospheric importance, but the mechanisms and dynamics are often unknown. Here, we explore these topics for a system with adjacent keto and enol chromophoric groups. The photochemistry in the S₁ and S₂ states of the most stable tautomer of 1,2-cyclohexanedione, is studied theoretically using molecular dynamics simulations with a semi-empirical excited-state potential. Results are compared with experiments. The main results are: (1) Calculations provide an interpretation of the measured absorption spectrum. (2) There is good agreement between the predicted and experimental photoproducts. (3) Agreement with experiments on the products suggests the latter can be predicted without treating non-adiabatic transitions. (4) The mechanisms of formation of the products are predicted by the simulations. (5) The adjacent keto-enol system is found to be photochemically very different from the pure keto one.