Electrochemical impedance spectroscopy has been used to determine the heterogeneous electron-transfer kinetic parameters for the reduction of the following nitrobenzene derivatives in acetonitrile containing 0.10 M tetrabutylammonium hexafluorophosphate at a mercury working electrode: 2-methylnitrobenzene(1), 2,6-dimethylnitrobenzene(2), 2,4,6-trimethylnitrobenzene(3), 2,3,5, 6-tetramethylnitrobenzene (4), pentamethylnitrobenzene (5), 2,4,6-triisopropylnitrobenzene (6) and 2,4,6-tri-tert-butylnitrobenzene (7). The results are discussed together with previously published results for nitromethane (8), nitroethane (9), 2-nitropropane (10) and 2-methyl-2-nitropropane (11). In general, a decrease in the standard electron-transfer rate constant, k(s), is seen on going from 1 to 7 and this trend continues for 8-11. Double layer effects are shown to be of minor importance. A previous explanation, based on increasing outer reorganization energies on going from 1 to 11 is called into question by the fact that the Gibbs energies of solvation of the radical anions of 1, 3 and 8 have been found to be almost identical. Arguments are presented supporting the idea that the outer reorganization energy should scale linearly with the Gibbs energy of solvation. A previously presented model, used to explain the effect of electrolyte cation size on the values of k(s), has been extended to include the effect of reactant size. It is inferred that the decrease in k(s), on going from 8 (CH3NO2) to 11 ((CH3)(3)CNO2) results at least partly from the increased size of the alkyl group, which causes the average distance of closest approach to increase from 8 to 11 resulting in an increased tunneling distance and smaller rate constants. This suggests that the "true" values of k(s), for 8-11, corrected for this distance effect, are roughly constant. Finally, semi-empirical molecular orbital methods (AMI; 6 and 7) and density functional theory (B3LYP; 1-5; 8-11) were used to estimate the contributions of the inner reorganization energy for 1-11. These contributions are small for 1 but steadily increase on going to 5. The inner reorganization contribution for 8-11 is quite substantial, ca. 4-5 kcal/mol (1 kcal/mol = 4.184 kJ/mol). This suggests that the inner reorganization term is a very significant factor underlying the sharp decrease in k(s) on going from 1 to 11. The principal structural change causing the calculated larger values of inner reorganization energy for 1-5 is the significant turning of the nitro group out of the plane of the ring in the neutral, a feature that is resisted in the radical anion. For 8-11, pyramidalization at nitrogen in the radical anion is the most significant structural change. (C) 2003 Elsevier B.V. All rights reserved.