ESR Studies of Monosulfide Radicals in Irradiated N-Acetyl-L-cysteine Single Crystals

Abstract

Electron spin resonance studies of three monosulfide radicals in N-acetyl-L-cysteine single crystals have been performed at 4 K. These radicals are characterized by very large g-tensor anisotropies; one is believed to have the largest g anisotropy ever reported for an organic free radical (1.9 ≤ g ≤ 2.5). Sulfur 3 d-orbital spin density is largely responsible for the unusual g tensor in at least two of these radicals. While these three radicals are almost identical in structure, their ESR parameters, such as g tensor and 33S hyperfine coupling constants, are affected by the environments of the radicals trapped in the crystal structure. Hydrogen bonding to the sulfur atom appears to play an important role.