DISPERSION SOLUTE-SOLVENT COUPLING IN ELECTRON-TRANSFER REACTIONS - I- EFFECTIVE POTENTIAL

Theories of electron transfer (ET) reactions and optical spectra in co ndensed phases consider electronic transitions between instantaneous B orn-Oppenheimer energies of the intramolecular electronic states which depend on the system nuclear configuration. With the aim of construct ing a molecular description of the solvent effect on these phenomena, we consider in the present paper a system composed of a polar polariza ble solute immersed in a solvent of polar polarizable molecules. The i nstantaneous free energies are defined in terms of partial partition f unctions obtained by averaging over the electronic degrees of freedom of the solute and the solvent. Electronic polarizabilities of the solv ent molecules are modelled as quantum Drude oscillators. For the solut e, two models are considered: (i) the Drude oscillator and (ii) the tw o-state solute. The former enables us to derive the solute-solvent dis persion potential with account for the effects of nonlocal polarizabil ity coupling in the solvent and the many-body solute-solvent dispersio n contributions. These effects are analyzed using equilibrium theories of nonpolar liquids. The two-state description of the solute involves redistribution of the electron density between the two localized site s. The instantaneous adiabatic (in contrast to diabatic in the Drude o scillator model) free energy can be derived in this case under the onl y restriction of the quantum character of the solvent electronic excit ations. It leads to the ET matrix element renormalized from its vacuum value due to the equilibrium field of the electronic solvent polariza tion and the instantaneous field of the permanent solvent dipoles. The theory predicts some useful relations which can be applied to treatin g the solvent effect on transition moments of optical spectra. The equ ilibrium ET matrix element is found to depend on the orientation of th e solute diabatic transition dipole in the solute molecular frame and the spectral shift due to solvation by permanent and induced dipoles. This offers an interesting phenomenon of self-localization of the tran sferred electron (zero ET matrix element). Finally, the comparison of two derivations performed enables us to write down the diabatic instan taneous free energies which can be used for a molecular formulation of the effect of the solvent and the solute energy gap on ET rates. (C) 1998 American Institute of Physics. [S0021-9606(98)03315-7].