ELECTRON-SPIN-RESONANCE INVESTIGATION OF THE CATION-EXCHANGE OF THE SEMIQUINONE OF 3,3',4,4'-TETRAHYDROXYBIPHENYL

ESR studies of the semiquinone of 3,3',4,4'-tetrahydroxybiphenyl in aq ueous ethanolic solutions have shown the radical undergoes cation exch ange 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 r = 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 derive d from positions with negative spin densities. The simulations also de monstrate that two slightly different radical conformers undergo excha nge simultaneously A temperature study from - 40 to 80 degrees C of on e sample, having [Na+] = 0.2 mol dm(-3), shows that the spltting const ants change with temperature, whereas the correlation time is found to be independent of temperature.