6800
J . Phys. Chem. 1988, 92, 6800-6807
in part to a change in configuration to a tetrahedral geometry cannot be determined from these results. Such evidence may be gleaned from future high-temperature spectrophotometric, Raman, NMR, or neutron-scattering experiments. Another consideration for establishing the configuration of the acetate complexes concerns whether this ligand is mono- or bidentate in character. Numerous X-ray crystallographic and spectroscopic data indicate that in the solid state acetate may serve as a bridging ligand,29,30a bidentate a monodentate ligand with partial secondary bonding,35 and a mondentate ligand.29,36v37However no corresponding results were found for ferrous complexes, and it cannot be. assumed that the same bonding will necessarily be preserved in solution. Certainly at 25 OC the formation constants for ferrous complexes with known bidentate ligands such as oxalate and malonate (log K1 = 3.02 and 2.24, r e s p e c t i ~ e l y )are ~ ~ somewhat larger than for acetate (1 .439),but this cannot be construed as definitive evidence for either mono-
or bidentate bonding. Association of carboxylates with alkaline-earth metal ions, on the other hand, is extremely weak (log K1 = 0 for a ~ e t a t e ) ~ Oand > ~ *is clearly indicative of a purely electrostatic interaction. This study demonstrates the relative strength of ferrous acetate complexes at high temperatures and suggests that acetate complexes must be considered seriously as vehicles for metal transport in both natural and man-made environments. Although acetate undergoes thermal decomposition at very high temperatures via heterogeneous catalysts,42its effective lifetime is ample for most practical scenarios.6 Migration of metals to create permeability for hydrocarbons and form ore deposits, as well as leaching of radioactive metal ions, particularly actinides, from nuclear repositories are important examples in hydrothermal systems. The greater relative strength of acetate complexes compared to chloride complexes must be of concern to power-generating plants, where corrosion and movement of radioactive cobalt in nuclear facilities are significant problems.
(29) Turpeinen, U.; Ahlgren, M.; Hamalainen, R. Acta Crystallogr., Sect. B 1982, 38, 1580. (30) Boukari, P. Y.; Busnot, A.; Busnot, F.; Leclaire, A,; Bernard, M. A. Acta Crystallogr., Sect. B 1982, 38, 2458. (31) Gorller-Walrand, C.; De Jaegere, S. Spectrochim. Acta, Part A 1972, 28, 257. (32) Faggiani, A.; Brown, I. D. Acta Crystallogr., Sect. B 1982,38, 2473. (33) Drew, M. G. B.; Hamid bin Othman, A,; Edwards, D. A,; Richards, R. Acta Crystallogr., Sect. B 1975, 31, 2695. (34) Simmons, C. J.; Alcock, N. W.; Seff, K.; Fitzgerald, W.; Hathaway, B. J. Acta Crystallogr., Sect. B 1985, 41, 42. (35) Hathaway, B. J.; Ray, N.; Kennedy, D.; OBrien, N.; Murphy, B. Acta Crystallogr., Sect. B 1980, 36, 1371. (36) Gadet, B. A. Acta Crystallogr.,Sect. B 1974, 30, 349. (37) Del Piero, G.; Cesari, M. Acta Crystallogr., Sect. B 1979, 35, 241 1. (38) Micskei, K. J . Chem. SOC.,Dalton Trans. 1987, 225. (39) Yatsirnirskii, K. B.; Fedorova, T. I . Zh. Neorg Khim. 1956, 1, 2310.
Acknowledgment. Financial support for this research was provided by the Office of Basic Energy Sciences, US.Department of Energy, under Contract DE-AC-05-840R21400 with Martin Marietta Energy Systems Inc. Helpful discussions with R. E. Mesmer were gratefully appreciated. Supplementary Material Available: Table of a summary of each titration experiment including those concentrations necessary to regress the results in terms of formation quotients (16 pages). Ordering information is given on any current masthead page. (40) Davies, C. W. J . Chem. SOC.1938, 277. (41) Topp, N. T.; Davies, C . W. J . Chem. SOC.1940, 87. (42) Palmer, D. A.; Drurnmond, S. E. Geochim. Cosmochim. Acta 1987, 50, 813.
Dynamic Solvent Effects in the Electron-Transfer Kinetics of S, Sianthryls Tai Jong Kang, Michael A. Kahlow, Dan Giser, Stephen Swallen, V. Nagarajan, Wbdzimierz Jarzeba? and Paul F. Barbara*it Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455 (Received: February 22, 1988)
Transient fluorescence experiments on bianthryl and a derivative of bianthryl have led to new insights into the excited-state SIintramolecular electron-transfer mechanism of compounds of this type. It has been found that the zero-order nonpolar and polar electron configurations that comprise the SI electronic wave function are strongly to moderately coupled. This coupling accounts for the small excited-state barrier for bianthryl, since the estimated matrix element for mixing of the configurations is comparable in magnitude to zero-order estimates of the barrier itself. Estimates have been made in the spirit of Marcus theory using spectroscopicestimates of the various energies, rather than a specific model for the solute/solvent interactions. In addition, subpicosecond measurements have been made on the electron-transfer kinetics. The measured rates are strongly correlated with measures of the solvent dynamics, but the electron-transfer relaxation times are not equal to the longitudinal dielectric relaxation times of the solvent. This is in contrast to the general behavior recently predicted for reactions of this type.
the reaction T~~ is roughly equal to the microscopic relaxation time Introduction T~ of the solvent in many cases.1*16 Recent the~reticall-~ and experimenta11*16 advances show that the quasi-equilibrium assumption of transition-state t h e ~ r y l ~ - ' ~ (1) Warshel, A,; Hwang, J.-K.. J . Chem. Phys. 1986, 84, 4938. as applied to electron-transfer reactions in solution is invalid in (2) Gertner, B. J.; Bergsma, J. P.; Wilson, K. R.; Lee, S.; Hynes, J. T. J . many cases. Instead, it appears that nonequilibrium features of Chem. Phys. 1987,86, 1377. the solute/solvent system, such as the dielectric dynamics of the (3) Sumi, H.; Marcus, R. A. J . Chem. Phys. 1986, 84, 4894. Rips, I.; solvent,20can play an important role. This is especially important Jortner, J. J . Chem. Phys. 1988, 88, 818. (4) Bixon, M.; Jortner, J. Faraday Discuss. Chem. SOC.1982, 74, 17. for small barrier (AG'
