Structural Effects on Intermolecular Electron Transfer Reactivity

Abstract

Rate constants (kij) measured by stopped flow are reported for 50 additional intermolecular electron transfer reactions between 0 and 1+ oxidation states of various compounds, enlarging our data set to 141 reactions between 45 couples in acetonitrile containing 0.1 M tetrabutylammonium perchlorate at 25 °C. Hydrazines with both saturated and unsaturated substituents, ferrocene derivatives, and heteroatom-substituted aromatic compounds are included in the couples studied. Least-squares fit of all the reactions to simple Marcus cross-reaction theory provides an internally consistent set of best fit intrinsic barriers ΔG‡ii(fit) (for self-electron transfer of each couple) covering a range of over 19 kcal/mol (rate constant range 2 × 1014) that predicts the kij rather accurately. All reactions have ratios of calculated to observed kij in the range 0.3−3.3 and 95% fall in the range 0.5−2.0. These results require that the preexponential factor for a cross reaction is close to the geometric mean of those for the self-reactions, which is not expected. Changes in internal reorganization energy (λv) have major effects on ΔG‡ii(fit), and changes in electronic overlap (Hab) have easily detectable ones, but the reactions studied are clearly not strongly nonadiabatic, even though in many cases the only electronic overlap at the transition state is between nonbonded alkyl groups. It is argued that these reactions occur in the “elbow region” between nonadiabatic and adiabatic electron transfer.