Chemically induced dynamic electron polarization (CIDEP) in moderately fast-relaxing systems

The relative intensities of individual hyperfine lines in the radical pair mechanism spin-polarized electron spin resonance spectrum of a radical are imperfectly calculated using standard theory, especially when the counter-radical undergoes fast relaxation. Here an approximate treatment valid in moderately fast relaxing systems is given in which the origin of the effect is treated as a Lorentzian distribution of spin mixing frequencies. This corresponds broadly to considering them to be determined by the lineshape of the electron spin resonance (ESR) spectrum of the counter-radical. The approach is shown to provide an adequate basis for simulation of the spin polarized spectrum of the hydroxymethyl radical produced, along with a fast relaxing acyl radical, on photolysis of hydroxypropanone. In principle it provides a measure of the relaxation time of the acyl through observations on hydroxymethyl. With ESR lines of normal linewidth any correction to existing theory is usually negligible within experiment...