ESR Study of a Single Crystal of L-Cystine Dihydrochloride Irradiated at Low Temperature

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Abstract
An electron spin resonance study has been made on free radicals produced in a single crystal of L‐cystine dihydrochloride by irradiation at liquid‐nitrogen temperature and on the process of their transformation during warming up to room temperature. All the spectra observed showed remarkable anisotropy in g factor, whereas the hyperfine structures were almost isotropic. From the analysis of the spectra the following conclusion has been drawn. The main primary radical produced at liquid‐nitrogen temperature is the L‐cystine dihydrochloride molecule with its S–S bond ionized probably to minus [R–CH2–S–S–CH2–R]—. At —72°C, the ionized bond is unstable and is ruptured to form the ``semistable'' radical R–CH2–S·* in which the direction of the C–S bond as well as the electronic structure remains almost unchanged from those of the parent radical. At room temperature the radical further transforms into the stable free radical R–CH2–S· in which the CH2–S group has rotated to a stable position accompanied by a change in the electronic state. The electronic structure for each radical is discussed in relation to the g factor analysis.