Comparisons between theoretical and experimental deuterium isotope effects for some outer-sphere electrochemical reactions

Experimental deuterium isotope effects upon the rates of outer-sphere electrochemical exchange reactions involving aquo and ammine complexes are compared with the predictions of contemporary theories which take into account nuclear tunneling from inner-shell vibrational modes. The isotope rate ratios calculated by considering nuclear tunneling of metal-ligand vibrational modes are in most cases much smaller than the observed rate ratios. Significant additional contributions probably arise from nuclear tunneling associated with internal ligand stretching modes, although this is estimated to be unlikely to account for the very large isotope effect observed for hexaaquochromium(III)/(II). The disparities seen between theory and experiment are speculated to be due to a contribution to the outer-shell reorganization energy from reorientation of hydrogen-bound solvent molecules. This conclusion is supported by the marked disparities seen between the relative isotope rate ratios for corresponding electrochemical and homogeneous reactions in comparison with the predictions of the dielectric continuum model.