Solvent effects on photoreactivity of valerophenone: a combined QM and MM study

Norrish type reactions for valerophenone in aqueous solution have been investigated by using the combined methods of DFT, CASSCF, and CASPT2 with molecular mechanics. It was found that formation of the intermolecular hydrogen bond in the complex of valerophenone with water results in a blue shift of the n,pi* excited states, while the Coulomb interaction between valerophenone and the bulk surrounding water is mainly responsible for the red shift of the pi,pi* excited states. As a result, the (3)pi pi* state becomes the lowest triplet state and is responsible for the long triplet lifetime observed for aqueous valerophenone. The (1)n pi*, (3)pi pi*, and (3)n pi* states were found to intersect in the same structural region, which appears to be the main reason why the intersystem crossing from (1)n pi* to (3)n pi* is very efficient for aqueous valerophenone and why most aromatic ketones have unique photophysical features such as a short singlet lifetime, high phosphorescence, and weak fluorescence. The Coulomb interaction between valerophenone and the bulk surrounding water has a significant influence on the alpha-C-C cleavage and the 1,5-H shift reaction. The 1,5-H shift is predicted to have a very small barrier on the triplet pathway and the alpha-C-C bond cleavages are not in competition with the 1,5-H shift reaction. This is in good agreement with the experimental findings that Norrish type II quantum yield is close to unity upon photoexcitation of aqueous valerophenone in the wavelength region of 290-330 nm.