Photochemical α cleavage of ketones as radiationless transitions

Theoretical and experimental study of the Norrish I photodissociation of aromatic ketones

A set of selected acetophenone derivatives was investigated using absorption and emission spectroscopy, laser flash photolysis and DFT calculations. The triplet state lifetimes and the activation energy of the cleavage reaction were measured. Computed triplet-triplet absorption spectra were found in very good agreement with the experimental ones. Bond dissociation energies, activation energies, partial charges, ground state geometries were calculated. The transition state theory TST was successfully used to calculate the cleavage rate constants: a very good correlation was found between the experimental and the calculated values. It is found that the entropy change influences the preexponential factor. This study also points out the role of the partial charges in the transition state, although this effect alone does not account for the reaction rate constants.