1340 from the reaction with CF,CH20H is that Hf transfer is exo- thermic by 15.6 kcal mol-' in the former reaction. This excess energy is deposited in the vibrational manifolds of the components of the complex of H+ transfer [(CF3),CDH/-OCH(CF,),1 which reduces the lifetime of the complex and separation occurs. The zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA J. Am. Chem. SOC. 1987, zyxwvu 109, 7340-7345 increased steric bulk of (CF,),CHOH compared to CF3CH20H may also contribute to the separation of this complex. zyxwvutsrqponmlkji Acknowledgment. We thank the National Science Foundation for support of this research. Pressure Effects on the Intervalence Transfer Electronic Absorption Band of the Mixed-Valence Bis(fulva1ene)diiron Monocation in Various Mediat Usha Sinha,'?' Michael D. Lowery,' William S. Hammack,'-' David N. Hendrickson,* and Harry G. Drickamer*'%' Contribution from the School zyxwvut of Chemical Sciences, Department of Physics, and Materials Research Laboratory, University of Illinois, Urbana, Illinois 61801. Received March 19, 1987 Abstract: The pressure dependence of the two intervalence transfer (IT) electronic absorption bands of the mixed-valence bis(fulva1ene)diiron cation is examined in various media. In general, a blue shift with increasing pressure is seen for both IT bands in all media. The blue shifting is attributable to an intramolecular effect, where an increase in pressure leads to an increase in the energy separation between the bonding ground and antibonding excited states of the IT transition. At high pressuras and to variable degrees in the different media (crystal lattice, PMMA polymer, intercalated into zeolite 13X) compression leads to intermoleculareffects on the energy of the two IT bands. Data obtained for the zeolite-intercalated bis(fulva1ene)diiron cation are analyzed in terms of the PKS vibronic model. Values for the electronic and vibronic coupling parameters are obtained from spectra measured in the range of 11-138 kbar. The value of the PKS electronic coupling factor e was shown to increase by a factor of zyxwvutsrqp - 1.5 over this pressure range. The degree of valence localization in the bis(fulva1ene)diironcation can be changed appreciably with pressure. From zyxwvutsr (ti and the PKS vibronic coupling factor X evaluated for the low-energy IT band it was calculated that lel/h2, a ratio that characterizes the valence localization, increases continuously from 0.59 at 11 kbar to 0.78 at 89 kbar. Pressure tuning spectroscopy (PTS) has been shown to be effective in studying the energy perturbations on valence orbitals resulting from pressure-induced compression of condensed phases.j Compression of crystals consisting of molecular units or large molecular ions leads to a reduction in interatomic and intermo- lecular distances which increases the overlap of valence orbitals. An electronic absorption band for a molecular moiety will shift to either higher or lower energy dependent on the balance between intramolecular and intermolecular effects of compression. In the case of intramolecular effects, an excitation from a bonding to an antibonding orbital should increase in energy with an increase in pressure. On the other hand, as a result of attractive van der Waals intermolecular interactions, a bonding to antibonding ex- citation localized on one molecular unit will generally shift to lower energy upon an increase in the pressure. This shift to lower energy as a result of intermolecular effects reflects the greater polariz- ability of the excited-state molecule with an electron in the an- tibonding orbital compared to the ground-state molecule. Very recently we reported4 the effect of pressure-induced freezing on the energy of the intervalence transfer (IT) electronic absorption band of binuclear mixed-valence complexes in solution. The energy of the IT band was monitored as the pressure was increased until, at pressures less than 10 kbar, the solutions froze. Essentially no shifts of the IT bands were observed, which is in marked contrast to the prediction of the dielectric continuum modeI5v6 for the solvent reorientation contribution to the energetics of the IT band. In view of the interesting results obtained for the IT bands of mixed-valencecomplexes in solution, PTS was applied to the study of IT bands of mixed-valence complexes in the solid state. The hemihydroquinone picrate salt of bis(fulva1ene)diironmonocation 'This work was supported in part by the Materials Science Division, Department of Energy under contract DE-AC02-76ER01198 and in part by National Institutes of Health Grant HL3652. 0002-786318711509-7340$01.50/0 (I), compound 1, was selected for this initial PTS study on solids with pressures up to 163 kbar. Fe Fe I Mixed-valence cation I has been the object of considerable The two iron ions in various salts of cation I (I) School of Chemical Sciences. (2) Department of Physics and Materials Research Laboratory. (3) (a) Drickamer, H. G. zyxwv Acc. Chem. Res. 1985, 18, 355. (b) Drickamer, zyxwvutsrq H. G. Annu. Reu. Phys. Chem. 1982,33, 25. (c) Drickamer, H. G. In?. Reu. Phys. Chem. 1982,2, 171. (d) Weber, G.; Drickamer, H. G. Q. Rev. Biophys. 1982, 16, 89. (4) Hammack, W. S.; Drickamer, H. G.; Lowery, M. D.; Hendrickson, D. N. Chem. Phys. Lett. 1986, 132, 231. (5) Hush, N. S. Prog. Inorg. Chem. 1967, 8, 391. (6) Powers, M. J.; Meyer, T. J. zyxwv J. Am. Chem. SOC. 1980, 102, 1289. (7) Rausch, M. D.; Kovar, R. F.; Kraihanzel, C. S. J. Am. Chem. SOC. (8) Cowan, D. 0.; LeVanda, C. J. Am. Chem. SOC. 1972, 94, 9271. (9) Mueller-Westerhoff, U. T.; Eilbracht, P. J. Am. Chem. Soc. 1972, 94, 9212. (10) LeVanda, C.; Bechgaard, K.; Cowan, D. 0.; Mueller-Westerhoff, U. T.; Eilbracht, P.; Candella, G. A,; Collins, R. L. J. Am. Chem. SOC. 1976, 98, 3181. (1 1) Mueller-Westerhoff, U. T.; Eilbracht, P. Tetrahedron Left. 1973, 1855. (12) LeVanda, C.; Bechgaard, K.; Cowan, D. 0. J. Org. Chem. 1976,41, 2700. (13) Morrison, W. H., Jr.; Hendrickson, D. N. Chem. Phys. Lett. 1973, 22, 119. 1969, 91, 1259. 0 1987 American Chemical Society