Recovery mechanism of the reaction intermediate produced by photoinduced cleavage of the intramolecular hydrogen bond of dibenzoylmethane

The recovery mechanism of the reaction intermediate (non-chelated enol form) produced by photoinduced cleavage of the intramolecular hydrogen bond of dibenzoylmethane was studied in various solvents by nanosecond laser flash photolysis. The recovery rate and mechanism depend strongly on the nature of the solvent. Unimolecular recovery of the intermediate to the chelated enol form takes place in acetonitrile, diethyl ether and dimethylsulphoxide with extremely small rate constants (1.1, 1.5 and 6.6 s(-1) respectively) despite the small activation energy (3.6 kcal mol(-1) in 3-methylpentane). The slow unimolecular recovery rate can be ascribed to the small frequency factor (7.0 X 10(5) s(-1)), i.e. the large negative entropy change for the formation of the chelated enol form. In non-polar aliphatic hydrocarbon solvents, a bimolecular recovery process via hydrogen-bond interactions between two intermediate molecules is included in addition to unimolecular recovery. In alcohols, a solvent-assisted recovery process by mutual hydrogen exchange between the intermediate and alcohol molecule(s) accelerates the recovery rate. Basic catalysts, e.g. KOH in ethanol and triethylamine in acetonitrile, increase the recovery rate considerably by an additional process through the enolate anion.