ENDOR study of x-irradiated single crystals of N-acetylglycine—A reinvestigation

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Abstract
Electronic and molecular structure of the stable radical produced by room temperature x‐irradiation of single crystals of N‐acetylglycine has been established from detailed Electron Spin Resonance (ESR) and Electron Nuclear Double Resonance (ENDOR) studies. Our findings indicate that the radical structure and the nature of the wave function of the unpaired electron are significantly different from those reported in a previous ’’negative’’ ENDOR study. In agreement with earlier ESR studies, we find that the stable radical at room temperature is CH3CONHĊHCOOH. ENDOR lines due to the α‐proton, three equivalent methyl protons, and six exchangeable protons have been identified and the hyperfine tensors determined. Spin density distribution in the radical has been compared with results of INDO–MO calculations. According to our findings the radical exists in four slightly different conformations. Isotopic substitution with 15N and deuteration of the exchangeable protons were helpful in the analysis of the complex ESR and ENDOR spectra. It is demonstrated that the radical concentration in the crystal has to be very carefully controlled for the success of an ENDOR investigation.