Comparison of different theories for slow-tumbling axial nitroxide ESR hyperfine centers in polycrystalline-amorphous substances

Abstract

The electron spin-resonance ESR hyperfine spectra computed by the Freed numerical–theoretical method have been critically compared with the recent analytical–theoretical results for axially symmetric nitroxide ESR centers undergoing isotropic orientational diffusion motion in polycrystalline-amorphous substances. Consistent with the new analytical–theoretical results, the dependencies of slow-motional line broadening and line shift on the associated orientational correlation time τ of the parallel-edge ESR centers determined from the Freed theoretical spectra are found to be τ−1/2 within numerical computational limits. In addition to this τ dependence, the absolute magnitudes of the slow-motional line broadening and line shift are in reasonable agreement between the two theories. Except for the occurrence of some unexplainable extra lines in the Freed theoretical spectra, their overall powder-pattern structures are consistent with those computed by the analytical–theoretical results.