This work is a theoretical comparative study on the adiabatic and non-adiabatic mechanisms of electrochemical reductions. In our previous paper [A. Ignaczak, W. Schinickler, Electrochim. Acta 52 (2007) 562 1] several aspects of the reaction kinetics were discussed for the adiabatic electron transfer from the electrode to the reactant, taking aquocomplexes as a testing case. In the present work the same reactions are studied in the non-adiabatic approach. First order perturbation theory is applied to analyze the rate dependence on quantum effects associated with innersphere nuclear vibrations using the same model Hamiltonian. It describes independently the solvent reorganization and the intramolecular modification of the reactant. In case of complexes the ligand shell in the oxidized and reduced states is characterized by two different frequencies, omega(1), and omega(2), respectively, so the ratio theta = omega(2)/omega(1), describes asymmetry of the potential energy surface. An influence of different theta values combined with high and low ligand frequencies on the rate enhancement due to the nuclear tunnelling is tested. The quantum correction, calculated as k(q)/k(s) (k(q) the non-adiabatic rated k, the adiabatic rate), is found to depend strongly on theta and omega(1) parameters as well as on the overpotential applied. The rate enhancement is especially sensitive to the initial state frequency omega(1), for systems characterized by larger theta values. For overpotential eta = 0 V, when CO, = 250 cm(-1) the ratio k,/k, is equal to 1.35 (theta = 2/3), 0.65 (0 = 1) and 0.38 (0 = 3/2), while for (omega(1)), = 1000 cm(-1) the corresponding values are 4.05, 3.94 and 4.77. These values increase significantly when the overpotential is applied, up to 14 for the largest values of omega(1), and theta. The transfer coefficient is also presented Cor the two reaction mechanisms as a function of all parameters tested. An effect of the reactant-electrode interaction and the friction parameter on the rate constants is also analyzed for all cases. (c) 2007 Elsevier Ltd. All rights reserved.