J. Am. Chem. Soc. zyxwvu 1991, 113, zyxwvu 9615-9620 9615 Experimental and Theoretical Study of Photoenolization Mechanism for 1 -Met h ylan t hraquinone Nina P. Gritsan,* Igor V. Khmelinski, and Oleg M. Usov Contribution from the Institute zyxwvutsr of Chemical Kinetics and Combustion, 630090 Novosibirsk, USSR. Received July 31, 1990. Revised Manuscript Received April 8, 1991 Abstract: Photoenolization of 1-methylanthraquinone (AQ) and its deuterated analogue (AQ-d,) has been studied by laser flash photolysis over a wide temperature range zyxwvutsr (120-340 K). Phototransfer of a H (or D) atom has been found to occur in both the singlet and triplet n r * 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-I ,IO-anthraquinone-I-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. Introduction Most of the photochemical reactions of organic compounds are as a rule nonadiabatic processes. However, there are quite a large number of exceptions to this general ru1e.I Most known adiabatic reactions occur zyxwvutsrq on the excited singlet surface of potential energy. Such processes are, for example, proton phototransfer via the hydrogen bond,2 and weak complexation (excimer and exciplex f~rmation).~ These adiabatic processes result in anomalous fluorescent properties, such as the substantial Stokes shift of fluorescence spectra2 or the luminescence of excited c~mplexes.~ Considerably less data have been reported on adiabatic photo- chemical reactions on the excited triplet energy surface. This is apparently associated with the fact that, in this case, it is very difficult to identify the adiabatic mechanism.' Recent examples of adiabatic photoreactions via the triplet state are the acyl group phototransfer in 1 -(acyloxy)anthraquinone derivatives4 and keto-enol tautomerism of 2-(2'-hydroxy- phenyl)benzo~azole.~ The most well-known triplet adiabatic (or by Turro's classification,1 "pseudoadiabatic") reaction is the photoenolization of o-alkylaryl ketones.6 Photoenolization has been also described for a-alkylq~inones.~-~ The triplet inter- mediate that precedes the formation of enol in the ground state is the excited triplet Intramolecular proton transfer in the excited singlet state occurs usually at very high rate constants (ca. 10l2 s-l) without activation (I) Turro, N. J.; McVey, J.; Ramamurthy, V.; Lechtken, P. Angew. Chem., Int. Ed. Engl. 1979, 18, 572. (2) (a) Weller, A. Nafurwissenschajen 1955.42, 175. (b) Klopffer, W. Ado. Photochem. 1977. zyxwvutsrqpo IO. 31 1. (c) Nagaoka, zyxwvutsrqp S.; Nagashima, U. Chem. Phys. 1989, 136, 153. (3) (a) Farster, Th.; Kasper, K. Z. Phys. Chem. (N.Y.) 1954, I, 275. (b) Walker, M. S.; Bednar, T. W.; Lumry, R. J. Chem. Phys. 1966,45, 3455. (c) Birks, J. B. Photophysics of Aromatic Molecules; Wiley: London, 1970. (4) (a) Gritsan. N. P.; Russkikh, S. A.; Klimenko, L. S.; Plyusnin, V. F. Teor. Eksp. Khim. 1983, 19, 455. (b) Gritsan, N. P.; Klimenko, L. S.; Shvartsberg, E. M.; Khmelinski, 1. V.; Fokin, E. P. J. Photochem. Phofobiol., A 1990, 52, 137. (5) Grellmann, K. H.; Mordzinski, A.; Heinrich, A. Chem. Phys. 1989, 136. 201. . - -, - - . (6) (a) Small, D. R.; Scaiano, J. C. J. Am. Chem. Soc. 1977, 99, 7713. (b) Haag, R.: Wirz, J.; Wagner, P. J. Helv. Chim. Acta 1977, 60, 2595. (c) Kumar, C. V.; Chattopadhyay, S. K.; Das, K. P. J. Ani. Chem. Soc. 1983, 105. 5143 . - - , - . . - . (7) Rommel, E.; Wirz, J. Helo. Chim. Acta 1977, 60, 38. (8) (a) Gritsan, N. P.; Rogov, V. A,; Bazhin, N. M.; Russkikh, V. V.; Fokin, E. P. Teor. Eksp. Khim. 1979, 15, 290. (b) Gritsan. N. P.; Bazhin, N. M. Izo. Sib. Old. Akad. Nauk SSSR 1979, 138. (c) Gritsan. N. P.; Shvartsberg, E. M.; Khmelinski. 1. V.; Russkikh, V. V. Zh. Fir. Khim. 1990, 64, 3081. (9) Grummt, U.-V.; Friedrich, M. Z. Chem. 1985, 434. (IO) Win, J. Pure Appl. Chem. 1984, 56, 1289. The rate constant for hydrogen-atom transfer in the excited triplet state of o-alkylaryl ketones has been estimated" as ca. 1Olo s-I. There is no quantitative data on a-alkylquinones. The aim of the present work was the detailed investigation of primary processes in the photoenolization of I-methylanthra- quinone. We have considered the following questions: (i) What are the roles of the singlet and triplet states in hy- drogen and deuterium phototransfer? (ii) Is the hydrogen and deuterium phototransfer a thermally activated process or may it be described as quantum tunnelling?I2 (iii) What are the geometry and electronic structure of the triplet enol intermediate? To answer these questions, we used not only experimental but also quantum-chemical methods that allowed us to determine the nature of the intermediates more definitively and to interpret experimental results. Experimental Section Apparatus. To take the spectra of products unstable at room tem- perature, the samples were irradiated at 87 K (or at 77 K) in glassy matrices by the light of a high-pressure mercury lamp through a glass filter (240 nm 5 X I400 nm). Absorption spectra were recorded on a Specord UV-vis spectrophotometer. The description of the setup for laser flash photolysis has been given previo~sly.'~ An excimer laser (10 ns, IO mJ, 308 nm) was used for irradiation. The probing system consisted of a double prismatic mono- chromator, a probing high-pressure xenon lamp, and a photomultiplier. Signals were recorded on an analogue-to-digital converter (20 MHz, &bit, 1024 channels) connected to a minicomputer. Kinetic parameters were determined by the method of nonlinear least squares.I4 Sample temperature was varied within 120-350 K accurate to f0.5 K with a thermostabilized nitrogen stream. To vary oxygen concentration, an argon-oxygen mixture was bubbled through the solution for 20 min. Oxygen concentration was estimated from reported values of the Henry coefficient. l5 Synthesis of Reactants. To obtain 1 -methylanthraquinone (AQ), o- (0'-toluy1)benzoic acid and polyphosphoric acid were fused for I h at 180 O C and cooled, and water was added. Then the product was extracted by ether. The extract was passed through a thin A1203 layer, concen- (11) (a) Laermer, F.; Elsaesser, T.; Kaiser, W. Chem. Phys. Lett. 1988, 148, 119. (b) Dick, B.; Ernsting, N. P. J. Phys. Chem. 1987, 91, 4261. (1 2) Formosinho, S. J. Chem. Soc., Faraday Trans. 2 1974, 70,605; 1976, 72, 1313. (1 3) Grivin, V. P.; Khmelinski, 1. V.; Plyusnin, V. F.; Blinov, 1. 1.; Balashev, K. P. J. Photochem. Photobiol., A 1990, 51, 167. (14) Johnson, K. J. Numerical Methods in Chemistry; Dekker: New York, 1983. (15! ,Kogan, V. B.; Fridman, V. M.; Kafarov, V. V. Reference Book on Solub~l~t~; Izd-vo AN SSSR: MOSCOW, 1961; Vol. 2, p 22. 0002-7863/91/1513-9615$02.50/0 0 1991 American Chemical Society