Misleading aspects of the viscosity effect on the heterogeneous electron transfer reactions

Some general and mutually intersecting problems of experimental studies and modeling the viscosity effects are considered in frames of the original study in sucrose and glucose aqueous solutions and reformulated in terms of the effect of non-Debye solvents. For the model reactants (Cr(III) and Co(III) complexes with ethylenediamminetetraacetate anion) the innresphere and solvent contributions to the total reorganization energy are computed on the basis of quantum chemical calculations at the DFT level. The Sumi-Marcus model is addressed to calculate the rate constants. An attempt of a similar analysis for non-aqueous solvents is made as well. It is concluded that the reliable estimate of the adiabaticity degree is not reducible to direct examination of the viscosity effect. This estimate is more realistic if one compares the reaction rates in the viscous carbohydrate solutions than in various pure solvents. (c) 2006 Elsevier B.V. All rights reserved.