4471
34 % yield ;
the latter is inaccessible using Na+Mn(CO)s- in TIIF.11n12
(11) M. D. Curtis, Inorg. Chem., 11,802(1972). (12) This work was supported by the National Science Foundation, the Alfred P. Sloan Foundation, and the Materials Science Center of Cornell University. We thank the Climax Molybdenum Co., a Division of American Metal Climax, Inc., for gifts of chemicals.
S. W. Ulmer, P. M. Skarstad J. M. Burlitch,* R. E. Hughes Department of Chemistry, Cornell University Ithaca, New York 14850 Received April 13, 1973
Triplet State of Ketones in Solutions. Quenching Rate Studies of Thioxanthenone Triplets by Flash Absorption' Sir: The quenching of the triplet air* state of aromatic ketones by the ground-state ketones (self-quenching) has been of interest in studies of triplet sensitization of photochemical reactions, especially because of the possible intervention of a triplet exciplex in the quenching Chapman and Wampfler first reported process. the interesting observation that triplet thioxanthenone underwent a self-quenching process. A self-quenching rate constant (ksq)of 6 X 10'O M-' sec-' in (CH3)3COH was obtained, based on thioxanthenone sensitization studies of 4,4-dimethyl-2-cyclohexenoneand assuming that triplet energy transfer was diffusion controlled. Recently, the ks, for thioxanthenone was redetermined by DeBoer and Schlessinger3 (using trans-stilbene as a triplet quencher) and was found to be (2.3 * 0.5) X IO9 M-' sec-l in benzene. The latter authors pointed out that energy transfer from triplet thioxanthenone (ET = 65 kcal mol-') to the enone (ET= 70 kcal mol-') should be considerably less than diffusion controlled. Based on known triplet energy transfer data,4 the k , for processes which are endothermic by 5 kcal mol-' can be predicted to be two or three orders of magnitude less than those of the diffusion controlled or to be -1 X lo7 M-' sec-' in this case. Taking thislower value for enone quenching of triplet thioxanthenone, and the data of Chapman and Wampfler,2 a k,, -2 X IO8 M-' sec-' in (CH&COH can be calculated. This recalculated value for k,, is ten times lower than the recently reported value in benzene3 and could imply a large solvent effect, a genuine inconsistency, or some other factor such as differences in purity of the thioxanthenone. A transient absorption at A, 625 5 nm was observed when a degassed solution of 5 X M thioxanthenone5in acetonitrile (620 * 5 nm in (CH&COH) was flash photolyzed.6 Monitoring at 600 nm, the 2 s 3
*
(1) Issued as National Research Council of Canada Contribution No. 13327. 91, 5390 (2) 0. L. Chapman and G. Wampfler, J. Amer. Chem. SOC., (1969). (3) C. D. DeBoer and R. H. Schlessinger, J . Amer. Chem. SOC.,94, 655 (1972). (4) (a) I
