ENDOR Studies in Molecular Crystals. II. Computer Analysis of the Polycrystalline ENDOR Spectra of Low Symmetry Materials

Abstract

The quantitative simulation of the electron nuclear double resonance(ENDOR)spectra of low‐symmetry, randomly oriented molecules is discussed with particular attention paid to those systems where electron cross relaxation effectively couples the spin packets of the electron paramagnetic resonance(EPR)spectrum.ENDOR transition frequencies are calculated employing the high field approximation; frequency shifts arising from g anisotropy and the SxIx and SyIy terms of the hyperfine Hamiltonian are then evaluated by perturbation theory. ENDOR signal intensities are considered to be determined by transition moments and by electron and nuclear spin relaxation processes. The ENDORspectra of polycrystalline samples of the x‐irradiated aliphatic diacids, HOOC(CH2) n COOH where n=1 to 8, and the alicyclic diacids, (CH2) m C(COOH)2 where m=2 to 4, have been recorded at 4.2°K and theoretical spectra computed. Reasonably good agreement is observed between experimental spectra and spectra computed assuming complete coupling of the spin packets by cross relaxation and isotropic relaxation rates. These assumptions are compatible with pulsed microwave measurements of electron relaxation rates.