NMR Study of Electron Transfer Rates and Spin Densities in p-Xylene and p-Diethylbenzene Anions

Abstract

Electron transfer reactions between a diamagnetic molecule and its paramagnetic anion modify the NMR spectrum in two different ways. First, the NMR lines are broadened and secondly, the lines are shifted. The line broadening may provide information on rates of electron exchange; the line shift is related to the spin density at the proton. The magnitude and the sign of the spin densities are quantities of great interest from a quantum-chemical point of view. It is useful, therefore, to provide methods for measuring spin densities. Usually they are derived from hfs parameters in ESR psectra. In this paper it is shown how they may be detected by the NMR line shifts. A special advantage of the NMR method is that the sign of the spin density is also easily available, because it is directly related to the direction of the line shift. Basic equations that describe both line broadening and line shifts are derived in this paper. The theory has been applied to p-xylene and p-diethylbenzene. Rates of electron transfer of p-xylene have been obtained from the broadening of the NMR lines. A combination of ESR and NMR has been used to obtain the values of the spin densities at the sites of the methyl and methylene protons; contrary to predictions in the literature, the sign is negative. An explanation for this is given on the basis of MO theory.