Electron Spin Resonance Study of γ-Irradiated Single Crystals of Semicarbazide Hydrochloride

Abstract

Electron spin resonance studies of γ‐irradiated single crystals of semicarbazide hydrochloride and the deuterated form, ND₂CONHND₃⁺·Cl⁻, revealed the presence of the stably trapped $·{\mathrm{N}}_{\mathrm{(\alpha )}}{\mathrm{H}}_{\mathrm{(\alpha )}}\mathrm{\hspace{0.17em}-\hspace{0.17em}}{\mathrm{N}}_{\mathrm{(\beta )}}{\mathrm{H}}_{\text{(β)3}}^{+}$ radical obtained by C–N bond rupture of the parent molecule. The principal $g$ values were $g_{\mathrm{aa}}{\text{\hspace{0.17em}=\hspace{0.17em}2.0041\hspace{0.17em}±\hspace{0.17em}0.0003, g}}_{\mathrm{bb}}\text{\hspace{0.17em}=\hspace{0.17em}2.0023,}\mathrm{and}{g}_{\mathrm{cc}}\text{\hspace{0.17em}=\hspace{0.17em}2.0044}$ . The principal elements of the hyperfine couplings were found to be ${\mathrm{N}}_{\mathrm{(\alpha )}}\text{\hspace{0.17em}=\hspace{0.17em}4.8\hspace{0.17em}±\hspace{0.17em}4.0, 34.2\hspace{0.17em}±\hspace{0.17em}1.0,}\mathrm{and}\text{11.8\hspace{0.17em}±\hspace{0.17em}4.0}\mathrm{G};{\mathrm{H}}_{\mathrm{(\alpha )}}\text{\hspace{0.17em}=\hspace{0.17em}−\hspace{0.17em}10.5\hspace{0.17em}±\hspace{0.17em}4.0, −27.6\hspace{0.17em}±\hspace{0.17em}1.0,}\mathrm{and}\text{−36.7\hspace{0.17em}±\hspace{0.17em}4.0}\mathrm{G};{\mathrm{N}}_{\mathrm{(\beta )}}\text{\hspace{0.17em}=\hspace{0.17em}−\hspace{0.17em}4}\mathrm{G};{\mathrm{H}}_{\mathrm{(\beta )}}\text{\hspace{0.17em}=\hspace{0.17em}15\hspace{0.17em}±\hspace{0.17em}0.5}\mathrm{G}$ . The ${\mathrm{N}}_{\mathrm{(\beta )}}$ coupling was estimated from the angular dependence of the linewidths. The direction cosines of the ${\mathrm{N}}_{\mathrm{(\alpha )}}{\mathrm{and\; H}}_{\mathrm{(\alpha )}}$ couplings were evaluated with reference to the orthorhombic crystallographic axes $\mathrm{(a,\; b,\; c)}$ . The direction cosines of the N–H bond calculated from the ESR spectra were found to agree with those obtained by x‐ray diffraction. The spin densities calculated for this radical indicate that the unpaired electron is highly localized on the $\alpha$ nitrogen, there being a spin density of 0.51 in the $\text{2pπ}$ orbital of the $\alpha$ nitrogen.