How important is parallel-mode EPR in electron spin echo envelope modulation studies of non-Kramers systems?

Integer-spin systems with S>1 sometimes exhibit a ground-state ‘non-Kramers (NK) doublet’ in their electron paramagnetic resonance (EPR) spectra. The preferred method of studying systems with this type of EPR signal has been to use a spectrometer in which the microwave field is parallel to the applied static field (parallel-mode EPR), rather than a traditional perpendicular-mode EPR spectrometer, in order to maximize the resulting NK-EPR signal. The efficacy of parallel and perpendicular mode electron spin echo envelope modulation (ESEEM) spectroscopy on the NK doublet of azido-hemerythrin (N3Hrred) are compared. These results demonstrate that for this technique, the advantages of parallel-mode over perpendicular-mode ESEEM are minimal at best. A simplified form of the theory underlying the analysis of NK-ESEEM is developed to explain the observations.