EPR line shifts and line shape changes due to spin exchange of nitroxide-free radicals in liquids 4. Test of a method to measure re-encounter rates in liquids employing 15N and 14N nitroxide spin probes.

EPR line shifts due to spin exchange of perdeuterated 2,2,6,6-tetramethyl-4-oxopiperidine-1-oxyl (14N-PDT) in aqueous solutions and the same probe isotopically substituted with 15N (15N-PDT) were measured from 293 to 338 and 287 to 353 K, respectively. Nonlinear least-squares fits of the EPR spectra yielded the resonance fields of the nitrogen hyperfine lines to high precision from which the shifts were deduced. The shifts are described by two terms: one linear and the other quadratic in the electron spin-exchange frequency, omegae. The quadratic term is due to spin exchange that occurs when two spin probes diffuse together and collide. A linear term is predicted for spin exchanges that occur upon re-encounter of the same two probes while they occupy the same "cage" before diffusing apart. The quadratic term has no adjustable parameters, while the linear term has one: the mean time between re-encounters, tauRE. The theory is cast in terms of the spin-exchange-induced line broadening that can be measured from each spectrum independently of the line shifts, thereby removing the explicit dependence of omegae on the temperature and the spin-probe concentration. In this form, theoretically, the value of the linear term is about a factor of 2 larger for 15N-PDT than for 14N-PDT for all temperatures; however, tauRE must be the same. Experimentally, we find that both of these expectations are fulfilled, providing strong support that the linear term is indeed due to re-encounter collisions. Values of tauRE derived from 14N-PDT and 15N-PDT are of the same order of magnitude and show the same trend with temperature as a hydrodynamic estimate based on the Stokes-Einstein equation.