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J . Phys. Chem. 1988, 92, 7174-7177
distance. The finite electronic mean free path, however, corresponds to a blurring of the Fermi surface, and this is obviously not included in calculations of the typical ion-ion potential. A crude estimate for a point ion model3* shows that such a blurring damps out the oscillatory behavior in the ion-ion interaction and makes at the same time its repulsive part harsher. This result is consistent with the observed correlation between the mean free path of electrons in expanded liquid cesium and the behavior of the structure factor data. The most surprising feature of the data in Figures 1 and 2 are the shapes of S(Q) and its density derivative (dS(Q)/dd), in the low-Q region for T = 1373 K and T = 1673 K (which are at reduced temperatures AT/T, of 0.29 and 0.13, respectively). At such large distances from the critical point insulating fluids (e.g., Ar) do not show such a marked enhancement in S(Q) for small Q. One explanation of such an effect, which has been discussed by Kahl and Hafner,32is that it reflects the strong density de-
pendence of the attractive part of the effective interionic interaction if screening is reduced as the metal-nonmetal transition is approached. Interestingly, the region of density where these enhancements occur is the same as that in which magnetic data2,3 indicate the presence of many-electron correlation effects. Acknowledgment. We are pleased to acknowledge valuable financial support by the Bundesministerium fur Forschung und Technology, Bonn. The neutron beam facilities made available for us by the Institute Laue-Langevin in Grenoble and by Professor P. Egelstaff in Chalk River enabled the realization of these experiments. Registry No. Cs, 7440-46-2. (31) Gaskell, T.; March, N. H. Phys. Lett. 1963, 7 , 169. (32) Kahl, G.; Hafner, J. Phys. Reu. A 1984, 29, 3310.
Long-Lived Quinone-Naphthalene Triplet Excited Complexes: Direct Measure of Rates of Proton Transfer within Intimate Radical Ion Pairs Guilford Jones, II,* and Nandini Mouli Department of Chemistry, Boston University, Boston, Massachusetts 02215 (Received: September 7, 1988) The quenching of the triplet state of chloranil (tetrachloro-p-benzoquinone)by derivatives of naphthalene in benzene results in the formation of relatively long-lived ( T = 100-350 ns) radical ion pair intermediates. For the electron donor naphthalenes having aliphatic side chain substitution,intrapair proton transfer can be directly monitored by laser flash photolysis. Measurement of radical ion pair (triplet excited complex) decay times and the quantum yields of resultant semiquinone (QH') radicals provide rate constants for decay via intersystem crossing (k,) and intra-ion-pair proton transfer (kH). The data reveal relative kinetic acidities of quinone-complexed radical cations and include activation parameters and isotope effects associated with the proton-transfer step. The coupling of electron transfer and proton transfer is a well-established alternative for delivery of the reducing equivalent of a hydrogen atom in organic photochemical systems.I The electron/proton-transfer route has been established for the reduction of carbonyl compounds, stilbenes, and other electrondeficient aromatics,l,* iminium ions,3 and q u i n ~ n e s . ~Several investigations of carbonyl photoreduction suggest that the combination of electron donors and acceptors in excited-state triplet quenching leads to intermediates (triplet exciplexes) with varying degrees of charge t r a n ~ f e r . ~These triplet exciplexes are seldom identifiable as true ion pair intermediates. In fact, in a number of flash photolysis studies of intermediates of this type, the transient signature is varied and sometimes confusing.6 In a few cases involving the photoreduction of carbonyl compounds (e.g., benzophenone/anilines), the primary intermediate appears as an ion pair, and the proton-transfer step, which is facilitated by the heightened acid-base properties of the radical ions, can be time resolved.' These reported phototransients which are involved in ( I ) (a) Lewis, F. D. Acc. Chem. Res. 1986, 19, 401. (b) Inbar, S.; Linschitz, H.; Cohen, S. G. J. Am. Chem. SOC.1980, 102, 1419. (c) Okada, T.; Karaki, I.; Mataga, N. Ibid. 1982, 104,7191. (d) Wagner, P. J.; Leavitt, R. A. Ibid. 1973, 95, 3669. (2) (a) Lewis, F. D.; Petisce, J. R. Tetrahedron 1986,42,6207. (b) Albini, A.; Spreti, S. Z . Naturforsch. 1986, 4 1 4 1286. (c) Arnold, D. R.; Wong, P. C.; Maroulis, A. J.; Cameron, T. S. Pure Appl. Chem. 1980, 52, 2609. (3) Mariano, P. S . Acc. Chem. Res. 1983, 16, 130. (4) (a) Bruce, J. M. In The Chemistry of Quinonoid Compounds; Patai, S., Ed.; Wiley: New York, 1974; Part I , Chapter 9. (b) Maruyama, K.; Furuta, H. Chem. Lett. 1986, 645. ( 5 ) (a) Wagner, P. J.; Truman, R. J.; Puchalski, A. E.; Wake, R. J. Am. Chem. SOC.1986,108, 7727. (b) Johnston, L. J. ; Scaiano, J. C.; Wilson, T. Ibid. 1987, 109, 1291. (c) Stone, P. G.; Cohen, S. G. Ibid. 1982, 104, 3435. (d) Kobashi, H.; Okada, T.; Mataga, N. Bull. Chem. Soc. Jpn. 1986,59, 1975. (6) For a summary of flash photolysis data on triplet exciplexes, see; Renge, I. V.; Kuzmin, V. A.; Borisevich, Yu. E. J. Photochem. 1985, 31, 67.
acid-base reactions are relatively short-lived (
