Electron-spin resonance investigation of the cation exchange of the semiquinone of 3,3′,4,4′-tetrahydroxybiphenyl

Abstract

ESR studies of the semiquinone of 3,3′,4,4′-tetrahydroxybiphenyl in aqueous ethanolic solutions have shown the radical undergoes cation exchange whenever the cation concentration [Na⁺] > 0.02 mol dm^–3. By the two-site exchange, which can be followed completely from no exchange ([Na⁺]= 0.02 mol dm^–3) to fast exchange, the coupling constants of three pairs of protons are interchanged. The fastest exchange observed leads to a correlation time τ= 1.43 ns.

From interactive simulations, based on the density-matrix method in the Alexander–Kaplan formalism, close matches are obtained for all observed spectra. The simulations unequivocally show that two splitting constants out of six are derived from positions with negative spin densities. The simulations also demonstrate that two slightly different radical conformers undergo exchange simultaneously. A temperature study from –40 to 80 °C of one sample, having [Na⁺]= 0.2 mol dm^–3, shows that the splitting constants change with temperature, whereas the correlation time is found to be independent of temperature.