General matrix ENDOR line shape model applied to the methyl radical in lithium acetate dihydrate using single crystal data

Abstract

A new general matrix ENDOR line shape model is tested against ENDOR data on the methyl radical in γ-irradiated lithium acetate dihydrate. All parameters in the line shape model are determined or narrowly limited by experiment. These include the dipolar tensors of all protons within 5 Å of the methyl radical, the microwave and radiofrequency magnetic fields, and the electronic and nuclear spin–lattice and spin–spin relaxation times. The theoretical simulations agree satisfactorily with the experimental lineshape, microwave magnetic field dependence, radiofrequency magnetic field dependence and angular variation of single crystal ENDOR line intensities. The effective nuclear spin–lattice relaxation time is ∼10 ms at 77 K and seems to be dominated by an angularly independent nuclear relaxation mechanism. The angularly dependent electron–nuclear dipolar interaction is found to be of much less importance for the ENDOR response. It is noted that the angular variation of ENDOR intensities is an important parameter to assess the nuclear relaxation mechanisms that control the ENDOR response. Data on cw and pulsed matrix ENDOR are compared and found to be similar. It is also shown that a model based on idealized line shape theory for disordered systems can satisfactorily reproduce matrix ENDOR line shapes. Both models allow calculation of the relative contributions from angularly independent and dependent ENDOR responses.