10264 J. Am. Chem. SOC. 1995,117, 10264-10275 Primary Isotope Effects on Excited State Hydrogen Atom Transfer Reactions. Activated and Tunneling Mechanisms in an ortho-Methylanthrone' Miguel A. Garcia-Garibay,*9$ Alla Gamamik: Ryan Bise: Lilis Pang," and William S. Jenks? Contribution from the Department of Chemistry and Biochemistry, University of Cal fomia, Los Angeles, Calijomia 90024, and Department of Chemistry, Iowa State University, Ames, Iowa 5001 1-31 11 Received May 12, 1 9 9 9 Abstract: Phosphorescence intensities and lifetimes of 1,6dimethylanthrone (1,CMAT) and 1 ,Cdimethylanthrone- d8 (1,CDMAT) were measured to determine the involvement of activated and quantum mechanical tunneling mechanisms (QMT) in their hydrogen and deuterium atom transfer reactions. The thermal-dependence of the radiative and thermal decay of the anthrone chromophore and the effect of methyl substitution were evaluated by using anthrone (AT), 2,3-dimethylanthrone, (2,3-MAT), and 10,lO-dimethylanthrone (10,lO-MAT). Measurements were carried out in methylcyclohexane (MCH) glasses between 18 and 80 K. The unreactive molecules AT, 2,3-MAT, and 10,lO-MAT present phosphorescence parameters typical of diarylketones with 3n,i7* configurations and show a relatively small temperature dependence changing from monoexponential at 77 K to nonexponential at the lowest temperature values. The phosphorescence intensity from 1 ,CDMAT was extremely weak and highly temperature- dependent. In contrast, no phosphorescence was detected in 1,4-MAT at all temperatures studied. Differences between deuterio and protio compounds were analyzed in terms of a large isotope effect on the hydrogen atom transfer reaction. A quantum mechanical tunneling mechanism was confirmed from nonlinear Arrhenius plots constructed with the average deuterium transfer rates of 1 ,CDMAT. A temperature-independent quantum mechanical tunneling reaction with a rate of 2 x lo3 s-I was calculated between 30 and 18 K. The involvement of reaction was confirmed by accumulation and detection of the photoenol product in ethanol glasses at all the temperatures studied. Changes in phosphorescence intensity observed even under conditions where the triplet lifetimes remain constant (18-30 K) were analyzed in terms of an avoided crossing mechanism predicted by orbital and state symmetry considerations. Introduction The intramolecular y-hydrogen abstraction by electronically excited carbonyl groups, also known as the Nomsh type-I1 reaction, is one of the most studied and best understood organic photochemical The reaction involves the transfer of a hydrogen atom from the y-carbon to the carbonyl oxygen to produce a 1P-biradical (Scheme la). In the case of arylalkylketones, the nature of the reactive excited state, its multiplicity, and a variety of structural, steric, electronic, and solvent effects have been evaluated.* Much attention has also been given to reactions involving ortho-alkyl-substitutedketones where an analogous process leads to formation of ortho- quinodimethanes in a reaction commonly referred to as photoen~lization.~-'~ Both reactions share hydrogen abstraction as the first step, but they differ in the following parts of the + Iowa State University. * University of California. @Abstract published in Advance ACS Abstracts, September 15, 1995. (1) Garcia-Gaxibay, M. A.; Gamamik, A.; Pang, L.; Jenks, W. S. J. Am. (2) Wagner, P.; Park, B.4. In Organic Photochemistry; Padwa, A. Ed.; (3) Wagner, P. J. Arc. Chem. Res. 1983, 16, 461-467. (4) Wagner, P. J.; Subrahmanyam, D.; Park, B. S. J. Am. Chem. SOC. (5) Wagner, P. J. Pure Appl. Chem. 1977, 49, 259-70. (6) Wagner, P. J.; Chen, C.-P. J. Am. Chem. SOC. 1976, 98, 239-241. (7) Smedarchina, Z.; Enchev, V.; Lavtchieva, L. J. Phys. Chem. 1994, (8) Porter, G.; Tchir, M. F. J. Chem. Soc. A 1971, 3772-3777. (9) Nakayama, T.; Honma, C.; Miki, S.; Hamanoue, K. Chem. Phys. Chem. SOC. 1994, 116, 12095-6. Marcel Dekker, New York, 1991; pp 227-366. 1991, 113, 709-710. 98, 4218-29. Lett. 1994, 213, 581-585. Scheme 1 mechanism. The Nomsh type-I1 reaction produces a triplet 1,4- biradical, while the corresponding species in the photoenoliza- tion reaction is identified as the triplet excited state of the photoenol product (Scheme lb). (10) Haag, R.; Wirz, J.; Wagner, P. J. Helv. Chim. Acta 1977, 60, 2595- (11)Gukrin. B.: Johnston. L. J. Can. J. Chem. 1989. 67. 473-480. 2607. (12) Findlay, D. M.; Tchir, M. F. J. Chem. SOC., Faraday Trans. I 1976, 72, 1096- 1100. (13) Das, P. K.; Encinas, M. V.; Small, R. D., Jr.; Scaiano, J. C. J. Am. (14) AI-Soufi, W.; Eychmuller, A.; Grellmann, K. H. J. Phys. Chem. Chem. Soc. 1979, 101, 6965-6970. 1991, 95, 2022-2026. (15) Gritsan, N. P.; Khmelinski, I. V.; Usov, 0. M. J. Am. Chem. SOC. 1991, 113, 9615-9620. 0002-786319511517-10264$09.0010 0 1995 American Chemical Society