Photoenolization of 1-methylanthraquinone (AQ) and its deuterated analogue (AQ-d,) has been studied by laser flash photolysis over a wide temperature range (120-340 K). Phototransfer of a H (or D) atom has been found to occur in both the singlet and triplet nr* states. The temperature dependence of the efficiency of the phototransfer of H and D atoms in the Inr* state has been analyzed. Piperylene quenching of AQ and AQ-d, triplet excited states has been studied. The rate constants of H- and D-atom phototransfer at room temperature have been estimated to be ca. 3 X IO'O s-I and ca. 1Olo s-I> respectively. Quantum-chemical calculations of potential energy surfaces and of electronic and geometrical structures of kcy intcrmediates have been performed by using the AM1 technique. A triplet o,?r-biradical has been found to be the intermediate preceding the formation of 9-hydroxy-1,10-anthraquinone-1-methide (AQM). It has been revealed that thermal transformation of the enol AQM to the initial quinone AQ can occur as an intramolecular process via reverse transfer of a H atom, or as a second-order reaction. The latter appears to involve the transfer of two H (or D) atoms in a collisional complex of two AQM molecules. The dependence of the rate constants of the intramolecular thermal transfer of H and D atoms on temperature and solvent nature has been analyzed.
JACS, 1991, V.113, No.25, pp.9615-9620