ERRORS IN MEASUREMENT OF ELECTRON-TRANSFER RATES BY MEANS OF ELECTRON SPIN RESONANCE

Rates of electron-transfer reactions can be conveniently determined from linewidths of electron spin resonance spectra in the limit of fast exchange by using an equation derived by Piette and Anderson (PA). Here the range of validity of this equation is investigated by a comparison of its predictions with exact linewidths obtained by numerical calculations. A correction term, depending on the second and fourth moments of the uncollapsed spectrum, is also derived which extends the range of validity of the PA fast-exchange equation. The ratio of the experimental linewidth to the square root of the second moment is a good indicator of the fast-exchange limit. This ratio must be less than 0.2 for the uncorrected fast-exchange equation to hold within 5%.