Aspects of aqueous iron and manganese (II/III) self-exchange electron transfer reactions

Ab initio methods were applied to the calculation of the reorganization energy A and the electronic coupling matrix element V _AB for the outer-sphere Fe(OH ₂) ₆ ^II/III and Mn(OH ₂) ₆ ^II/III self-exchange electron transfer (ET) reactions. For the Fe case, we find an appreciable effect on V _AB depending on whether the minority spin electron occupies the d _xy orbital or a mixture of d _xz/d _yz orbitals in the Fe ^II ion. While these two possible nearly isoenergetic electron accepting states alter the magnitude and distance dependence of V _AB, they do not affect the internal reorganization energy λ _I to any significant level. The magnitude and distance dependence of V _AB are found to be strongly dependent on encounter orientation, as expected. V _AB values for corner-to-corner encounter orientations are substantially larger at any given ET distance considered than those for face-to-face encounter orientations. Values of the decay parameter β are in good agreement with well-accepted values. The adiabaticity criterion is tied to orientation and distance dependence of V _AB.