Experimental and theoretical study of photoenolization mechanism for 1-methylanthraquinone

Article abstract of DOI:10.1021/ja00025a029

Photoenolization of 1-methylanthraquinone (AQ) and its deuterated analogue (AQ-d₆) 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 nπ* states. The temperature dependence of the efficiency of the phototransfer of H and D atoms in the ¹nπ* state has been analyzed. Piperylene quenching of AQ and AQ-d₆ 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 × 10¹⁰ s^-1 and ca. 10¹⁰ s^-1, respectively. Quantum-chemical calculations of potential energy surfaces and of electronic and geometrical structures of key intermediates have been performed by using the AMI technique. A triplet σ,π-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.