Deuterium isotope effects on the photoreduction of 9,10-phenanthrenequinone and benzophenone by 2-propanol

Reactive rate constants for the quenching of the triplet state of 9,10-phenanthrenequinone by 2-propanol were measured using laser flash photolysis and deuterium isotope effects for the abstraction of the alpha hydrogen (deuterium) were determined. The values of kH/kD were 1.69, 1.45, and 1.36 in carbon tetrachloride (CCl4), chlorobenzene (CB), and acetonitrile (MeCN), respectively. These results are in agreement with a mechanism involving initial hydrogen abstraction and eliminate a mechanism involving initial electron transfer.This same reaction was studied for the triplet of benzophenone. Values of kH/kD were 2.56, and 2.28, in CCl4, and MeCN, respectively. The rate of hydrogen abstraction was faster in MeCN than in CCl4. These results are explained considering a polar transition state which is stabilized by polar solvents.