An ab initio study of specific solvent effects on the electronic coupling element in electron transfer reactions

Abstract

Specific solvent effects on the electronic coupling element for electron transfer are examined using two model donor-acceptor systems (Zn 2 ϩ and Li 2 ϩ ) and several model ''solvent'' species (He, Ne, H 2 O, and NH 3 ). 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 H 2 O and NH 3 significantly increase H ab . The distance dependence ͑essentially exponential decay͒ is weakly affected by a single intervening solvent atom-molecule. However, when the donor-acceptor distance increases in concert with addition of successively greater numbers of solvent species, the decay with distance of H ab is altered appreciably. Effects due to varying the orientation of molecular solvent are found, somewhat surprisingly, to be quite modest.

A Theoretical Study of the Electronic Coupling Element for Electron Transfer in Water

“…28,29 None of the coupled molecular dynamics/quantum mechanical studies discussed above considers all-electron models for the solvent. Our previous ab initio results 23 suggest that this may be a significant omission. Although the geometries examined in our studies were idealized, we found little orientation dependence for the superexchange effects induced by water, and we obtained evidence to suggest that water's effects were due largely to hole-type superexchange (i.e., the electronic coupling was propagated to a large degree through the occupied orbitals of water).…”

“…23 In the ab initio results the systems were described using complete active space self-consistent field (CASSCF) wave functions; 40 the calculations were performed using MOLCAS 3. 41 Since ) for Zn2 + with (solid line, circles) and without (dashed line, squares), water present.…”

“…The basis sets used have been calibrated in a previous study 23 and have been shown to be accurate to within 10-20% for the systems examined here, relative to results obtained from expanded basis sets.…”

“…22 Two recent studies have also examined the effects of multiple waters on the decay of H ab with distance, using idealized geometries for sequential addition of waters. 15,23 In these two studies it was found that water reduced the rate of decay of H ab with distance and that neither the size nor the decay of H ab with distance were significantly affected by the water orientation between the donor and acceptor. Theoretical studies of solvent effects on H ab for the C-clamp compounds of Zimmt and co-workers 17 have also been performed.…”

“…23,30 Following discussion of the solvated Zn 2 + results several other donoracceptor pairs (Li 2 + , Be 2 + , Na 2 + , Mg 2 + , Cu 2 + ) are examined at the semiempirical and/or ab initio level in order to assess the donor-acceptor energy-dependence of the conclusions drawn from the Zn data.…”