Solvent dynamical effects in electron transfer: electrochemical-exchange kinetics of sesquibicyclic hydrazines as a probe of coupled vibrational activation

Abstract

Abstract : Rate constants for electrochemical exchange of three sesquibicyclic hydrazine-radical cation redox couples, k(e)(ex), obtained at mercury by means of phase-selective ac voltammetry, are reported in seven solvents - acetonitrile, acetone, nitromethane, N, N-dimethylformamide, dimethylsulfoxide, benzonitrile and methanol. The hydrazine couples - denoted here 1+/0, 2+/0, and 3+/0 - feature variations in the bridgehead alkyl groups; they each exhibit substantial high-frequency vibrational barriers (4.5-5 kcal mol-1) to electrochemical exchange. These system's therefore provide an interesting opportunity for the assessment of the manner and extent in which overdamped solvent relaxation may limit the electron-transfer dynamics under such circumstances. As in previous studies (ref. 2), scrutiny of the solvent- dependent rate constants enable at least semiquantitative estimates of the degree to which the kinetics are controlled by solvent dynamics as well as activation energetics. In addition to the usual dielectric-continuum treatment, estimates of the latter outer-shell component required for the solvent-dynamical analysis were required from an observed SESQUIBICYCLIC HYDRAZINES, ELECTRON- TRANSFER DYNAMICS, ELECTROCHEMICAL-EXCHANGE KINETICS.