AN AB-INITIO STUDY OF SPECIFIC SOLVENT EFFECTS ON THE ELECTRONIC COUPLING ELEMENT IN ELECTRON-TRANSFER REACTIONS

Specific solvent effects on the electronic coupling element for electr on transfer are examined using two model donor-acceptor systems (Zn-2( +) and Li-2(+)) and several model ''solvent'' species (He, Ne, H2O, an d NH3). The effects are evaluated relative to the given donor-acceptor pair without solvent present. The electronic coupling element (H-ab) is found to depend strongly on the identity of the intervening solvent , with He atoms decreasing H-ab, whereas H2O and NH3 significantly inc rease H-ab. The distance dependence (essentially exponential decay) is weakly affected by a single intervening solvent atom-molecule. Howeve r, when the donor-acceptor distance increases in concert with addition of successively greater numbers of solvent species, the decay with di stance of H-ab is altered appreciably. Effects due to varying the orie ntation of molecular solvent are found, somewhat surprisingly, to be q uite modest. (C) 1998 American Institute of Physics. [S0021-9606(98)30 141-5].