Electron transfer in bis(hydrazines), a critical test for application of the Marcus model

Electron transfer in the cations of bis(hydrazines), bridged by six different pi -systems (compounds 1-6) is studied using ab initio and density functional theory (DFT) methods. Due to ionization from an antibonding combination of the lone-pair orbitals of the nitrogens in one of the hydrazine units, conjugation is introduced in the N-N bond of that unit. This leads to a shortening of the N-N bond distance and an increase of the planarity around the nitrogens. Due to steric hindrance, this causes an increase of the angle, called phi, between the lone-pair orbital on the nitrogen attached to the bridge and the p-orbital on the adjacent bridge carbon for the ionized unit in the charge localized, relaxed state of the molecule. This angle controls the magnitude of the electronic coupling. In the fully delocalized symmetric transition state of the ion, however, this angle is low for both units, due to the fact that the conjugation introduced at the ionized hydrazine unit is now shared between both units. An extended pi -system is formed including the orbitals of the hydrazine units and the bridge, which leads to a large electronic coupling. The electronic coupling derived by optical methods, corresponding to the structure of the relaxed asymmetric cation with a large phi for the ionized unit, appears to be much smaller. We believe this is due to an approximate cosine dependence on phi of the coupling. The calculations carried out support these conclusions. (C) 2001 John Wiley & Sons, Inc.