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Reviews in Computational Chemistry Volume 18

Reviews in Computational Chemistry Volume 18

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206 Charge-Transfer Reactions <strong>in</strong> Condensed Phases<br />

of the ET free energy surfaces to the solvent-<strong>in</strong>duced component of the optical<br />

Franck–Condon provides a unique possibility to apply the statistical mechanical<br />

analysis of ET and CT energetics and to test it on experiment. The band<br />

shape analysis of optical profiles is thus the key factor <strong>in</strong> a successful <strong>in</strong>terplay<br />

between theory and experiment.<br />

This chapter outl<strong>in</strong>es some recent advances <strong>in</strong> the statistical mechanical<br />

analysis of the CT energetics. The basic strategy used <strong>in</strong> this approach is to<br />

<strong>in</strong>troduce new physical features of CT activation <strong>in</strong>to the system Hamiltonian<br />

used to build the free energy surfaces. These are then applied to calculate the<br />

Franck–Condon factors and determ<strong>in</strong>e how the changes <strong>in</strong> the physics of the<br />

problem affect the optical observables. This development highlights two fundamental<br />

results. First, the MH model of fixed charges solvated <strong>in</strong> a dense,<br />

condensed-phase environment leads to a very accurate representation of the<br />

ET energetics <strong>in</strong> terms of two <strong>in</strong>tersect<strong>in</strong>g parabolas. The static nonl<strong>in</strong>ear solvation<br />

effects are generally weak and do not substantially distort the parabolas.<br />

There is, however, ample room to modify the free energy surfaces when<br />

changes <strong>in</strong> the electronic density of the donor–acceptor complex are allowed<br />

either through polarizability or electronic delocalization. The CT free energies<br />

then <strong>in</strong>herit nonl<strong>in</strong>ear features, and a number of <strong>in</strong>terest<strong>in</strong>g consequences for<br />

optical observables can be anticipated. These fasc<strong>in</strong>at<strong>in</strong>g phenomena will be<br />

the subject of future research.<br />

ACKNOWLEDGMENTS<br />

D.V.M. acknowledges the support by the Department of <strong>Chemistry</strong> and Biochemistry at<br />

ASU and partial support by the Petroleum Research Fund, adm<strong>in</strong>istered by the American Chemical<br />

Society, (36404-G6). G.A.V. acknowledges support from the Department of Energy, Basic Energy<br />

Sciences Program.<br />

REFERENCES<br />

1. R. A. Marcus, Adv. Chem. Phys., 106, 1 (1999). Electron Transfer Past and Future.<br />

2. P. F. Barbara and W. Jarzeba, Adv. Photochem., 15, 1 (1990). Ultrafast Photochemical<br />

Intramolecular Charge and Excited State Solvation.<br />

3. B. Bagchi and N. Gayathri, Adv. Chem. Phys., 107, 1 (1999). Interplay Between Ultrafast<br />

Polar Solvation and Vibrational Dynamics <strong>in</strong> Electron Transfer Reactions: Role of High-<br />

Frequency Vibrational Modes.<br />

4. B. Bagchi and R. Biswas, Adv. Chem. Phys., 109, 207 (1999). Polar and Nonpolar Solvation<br />

Dynamics, Ion Diffusion, and Vibrational Relaxation: Role of Biphasic Solvent Response <strong>in</strong><br />

Chemical Dynamics.<br />

5. F. O. Ra<strong>in</strong>eri and H. L. Friedman, Adv. Chem. Phys., 107, 81 (1999). Solvent Control of<br />

Electron Transfer Reactions.<br />

6. N. S. Hush, Progr. Inorg. Chem., 8, 391 (1967). Intervalence-Transfer Absorption. Part 2.<br />

Theoretical Considerations and Spectroscopic Data. See also, R. S. Mulliken and W. B.<br />

Person, Molecular Complexes, Wiley, New York, 1969.

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