Paramagnetic Resonance in L-Cysteine Hydrochloride Irradiated at 77°K

An electron paramagnetic resonance study has been made on a single crystal of L‐cysteine hydrochloride irradiated with a 1.5‐MeV electron beam at 77°K. The hyperfine structure of the main resonance pattern was found to be an almost isotropic double doublet, whereas the g factor was found to be highly anisotropic but axially symmetrical, the principal values being 2.29, 1.99, and 1.99. The main resonance has been attributed to the free radical $$\begin{array}{c}\mathrm{HOOC}–\mathrm{CH}–{\mathrm{CH}}_2–\mathrm{S}.\\ \mathrm{\hspace{0.17em}\; \hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}–}\\ \mathrm{\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}}{\mathrm{NH}}_3\mathrm{Cl}\end{array}$$ with the unpaired electron occupying one of the two nearly degenerate orbitals of the sulfur atom. A remarkable broadening of the resonance spectrum was observed at elevated temperatures, and evidence is given that this broadening is due to a short spin—lattice relaxation time due to a large spin—orbit coupling in this free radical.