Endor induced electron paramagnetic resonance: Application to the resolution of overlapping spectra

Abstract

A technique for resolving overlapping EPR spectra is demonstrated in a study of X- and γ-irradiated crystals of 1,1-cyclobutanedicarboxylic acid. Discrimination of overlapping spectra is accomplished by recording the electron paramagnetic resonance desaturation induced by a radiofrequency field which excites nuclear spin transitions of only one of the several paramagnetic species present. This spectrum, referred to as ENDOR induced electron paramagnetic resonance (EI-EPR), is the difference between the ordinary partially saturated EPR spectrum and the EPR spectrum recorded in the presence of a resonant radiofrequency field. An instrumental arrangement for the fast and convenient measurement of EI-EPR at several Zeeman modulation frequencies is described. The technology is employed to resolve the complicated EPR spectrum which arises from three distinct radical species existing in radiation damaged 1,1-cyclobutanedicarboxylic acid. The EI-EPR, electron nuclear double resonance, and electron electron double resonance spectra of the alicyclic and aliphatic radical are recorded as a function of temperature and crystal orientation. EI-EPR is demonstrated to permit resolution of EPR spectra due to different radical species, different molecular conformations of a given species, and radicals in different (magnetically inequivalent) sites in the crystal unit cell. All alicyclic radicals exhibit temperature dependent beta hyperfine interactions although alpha and gamma interactions show little variation with temperature. EI-EPR signal intensities are found to vary with nuclear transition probability, relaxation parameters which characterize the various transitions, and with the applied de magnetic field (i.e., the nuclear Zeeman interaction).