Effects of Hydrostatic Pressure and Temperature on Spin Exchange between Free Radicals in Solution

The paramagnetic resonance linewidths of the di-tert-butyl nitroxide free radical have been measured as a function of hydrostatic pressure and of temperature in the pure hydrocarbon solvents methylcyclohexane, n-pentane, and propane. These data are reported at conditions under which intermolecular spin exchange gives the principal contribution to the linewidth. Deviations of the linewidth from a linear dependence on T/η, where η is the liquid viscosity, are attributed to a dropping off of p, the average exchange probability per radical—radical encounter, from unity at low values of η/T. The viscosity and temperature dependence of p is found to be p=1—exp(—aη/T), where a is a positive constant which depends upon the solvent. This form of p(η,T) is shown to arise from a theory which treats the exchange reaction as a unimolecular kinetic process which may occur, however, only during a radical—radical encounter in the liquid. Radical—oxygen exchange rates were also measured in methyl cyclohexane and the same theory explains the η and T dependence.