Modern developments and prospects in multifrequency High Field EMR: High Frequency ENDOR spectroscopy

The most detailed information on paramagnetic systems is obtained from studies in single crystals. It is quite obvious the advantage of being able to extract the same amount of information from studies in polycrystalline samples or frozen solutions. Regarding nuclear hyperfine couplings, this is made possible by using ENDOR spectroscopy at X and Q band only in the case of systems with large g tensor anisotropy. The method, based on the selective saturation of different manifolds of molecular orientations, has been and is applied with success by many authors for studying metalloproteins and metalloenzymes. Due to the enhanced orientation selection, ENDOR at much higher field/frequency than X and Q band allows applying the same experimental approach to investigate the important research field of organic and bioorganic radicals with small g tensor anisotropy, even much smaller than that of nitroxide radicals. The present study on an indolinone nitroxide provided evidence about the potential of ENDOR spectroscopy at 220 GHz. In addition to the magnitudes of the proton hyperfine tensors, the orientations of the tensor axes with respect to the g tensor principal system were determined from simulation of the high field ENDOR spectra. The tensor values obtained in this way have a great accuracy, since they represent the set which best reproduce all the ENDOR spectra recorded across the EPR spectrum. Moreover the spectra are characterized by a high degree of orientation selectivity, as proven by the low value (0.3-0.5 mT) of the parameter  (width of the saturated portion of the EPR spectrum).