ENDOR Measurements of Proton Hyperfine Couplings

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Abstract
ENDOR (electron‐nuclear double resonance) spectroscopy was used to measure hyperfine couplings in radicals in which the unpaired spin density is concentrated mainly on atoms two bond lengths removed from the proton. The measurements were made on radicals produced by x irradiation of single crystals. The anisotropic part of the hyperfine coupling, as a function of the isotropic coupling, was obtained for two types of radicals: (1) neutral radicals obtained by irradiation of certain carboxylic acids and amino acids at room temperature in which the unpaired spin density is localized primarily in a π2p carbon orbital, and (2) radical anions obtained by irradiation of certain carboxylic acids and amino acids at 4.2°K in which the unpaired electron is localized primarily in a π2p carbon orbital on the carboxyl group.