Nuclear Transverse Relaxation in Electron-Transfer Reactions

Abstract

The problem of proton magnetic resonance linewidths for molecules that are taking part in one electron transfer reactions is re‐examined, and it is concluded that the ``weak‐pulse'' limit (a<sub>H</sub>τ<sub>p</sub>≪1) is more generally applicable than the ``strong‐pulse'' limit (a_Hτ_p≫1) that was discussed by McConnell and Berger. Density matrix equations are set up using the formalism of Alexander in order to calculate line shapes from the hyperfine splitting constants and the lifetimes of the diamagnetic and paramagnetic species; and it is shown that when more than one absorption line are observed, under certain conditions, the bimolecular rate constant for electron transfer may be determined without a knowledge of the concentration of unpaired electrons. These equations are applied to the electron transfer reaction between NNN′N′‐tetramethyl‐p‐phenylene diamine and its monocation radical in acetonitrile, and we find that the rate constant for electron transfer is (2.0±1.0)×10⁸ liter mole⁻¹ sec⁻¹.