3206
J. Phys. Chem. 1981, 8 5 , 3206
TABLE I: Calculated and Experimental Values of R 580 Ya 48 model 1 2.52 model 2 2.55 model 3 2.50 A,
nm
exptlC a
2.0
f
590 48 2.52 2.55 2.50 0.5 2.5:
Reference 1.
t::
600 35 2.61 2.81 2.70 2.5:
610 20 2.87 3.81 3.38
620 10 3.73 13.5 7.24
io2 2.9 1" 11 :;Ob
Based on data of ref 1.
This work.
dependent solely on the R value at 620 nm, the most experimentally inaccessible and uncertain value. Furthermore, the reevaluation of the data, using a technique that allows for the occurrence of systematic as well as random errors, supports a quartet reactivity model in this system. The relevant conclusions are summarized in Table I which shows that the rather different R value for 620 nm of this work is seen to be consistent with models 2 and 3 invoking back-intersystem crossing, whereas the value required for model 1lies just outside the estimated range of R. It is unfortunate that the uncertainties are so large, but at the present state of these experiments it is not likely that better 620-nm data could be obtained. Also there are other uncertainties to be considered, among these uncertainties in y (ratio tpr~ud/treectanJ8and the probability that some excited doublet-state absorption persists at 620 nma9 With the information available there seems little basis on which to choose between the two quartet reactivity models or some intermediate reality. Model 3 requires a back-intersystem crossing yield of unity, thus identifying the doublet/quartet level spacing with the apparent activation energy12of 10 kcal mol-l for the emission lifetime decrease. For model 2, fbi, < 1so that the emission lifetime would depend on two rate constants, implying a nonlinear Arrhenius plot. That this need not be in contrast to experiment, however, is readily shown by a proper kinetic analysis. Finally, the doublet reactivity model' unambiguously predicts a greater than twofold increase in photochemical quantum yield on irradiation into the doublet absorption band, which, particularly in [Cr(en)#+, is quite accessible and distinct from the quartet band. Such a yield increase has not been observed in the relevant experimental in(8) Other workers have measured somewhat different y values than quoted in ref 1: R. G. Linck, personal communication. (9) Excited doublet-stateabsorption spectra for Cr(II1)complexes are broad,lOJ1and it is unlikely that 6 decreases from 4 at 560 nm to zero at 620 nm as claimed.' (IO) T.Ohno and S. Kato, Bull. Chem. SOC.Jpn., 43,8 (1970). (11) A. D.Kirk, P. E. Hoggard, G. B. Porter, M. G. Rockley, and M . W. Windsor, Chem. Phys. Lett., 37, 199 (1976). (12)R.T.Walters and A. W. Adamson, Acta Chem. Seand., Sect. A, 33, 53 (1979). (13) E. E.Wegner and A. W. Adamson, J. Am. Chem. Soc., 88,394 (1966). (14) S. N.Chen and G. B. Porter, Abstracts of Xth Informal Conference on Photochemistry, Oklahoma State University, Stillwater, OK, 1972.
0022-3654/81/2085-3206$01.25/0
ve~tigations.'~J~ This is therefore also a major obstacle to belief in the doublet reactivity model and further supports the quartet reactivity analysis presented here. Acknowledgment. The author thanks Dr. A. W. Adamson for the use of the laser equipment, for access to the earlier data, for financial support, and for stimulating discussion and debate of this question. He also thanks Marina Larsen and Alistair Lees for instruction and advice on the use of the laser system. Supplementary Material Available: At the request of the Editor, this manuscript was abbreviated, necessitating removal of the kinetic analysis and the details of the data reevaluation. The full manuscript is available as supplementary material (15 pages). Ordering information is available on any current masthead page. Department of Chemistry University of Victoria Victoria, British Columbia, Canada V8 W 2Y2
A. 0. Kirk
Received: March 25, 1980; In Flnal Form: Aprll6, 1981
Reply to the Comment by Kirk. The Case of Two Reactive and Intercommunicating Exclted States
Sir: The exact kinetics of a pulsed laser experiment producing two potentially reactive and potentially intercommunicative states are examined in detail, with special reference to aqueous Cr(en)l+. The various possible regimes of rate constant values in all cases but one lead to physically questionable absolute values. The acceptable regime is that in which the first doublet thexi state is directly chemically reactive. A dissent to the reading of earlier results on the monitoring of the rate of appearance of the primary photoproduct is discussed. Supplementary Material Available: At the Editor's request, only an abbreviated version of this reply to the comment by Kirk is published in the printed edition of the Journal. The complete text is available as supplementary material (21 pages). Ordering information is available on any current masthead page. Department of Chemistry University of Southern California Los Angeles, California 90007
Arthur W. Adamson*
Baffelle Pacific Northwest Laboratory Richland, Washington 99352
Robert C. Fukuda
Chemistry and Chemical Engineering University of Saskatchewan Saskatoon, Saskatchewan Canada S7N OW0
R. Torn Walters
Received: June 18, 1980; In Final Form: Aprll29, 1980
0 1981 American Chemlcal Society
