EPR and ENDOR studies of single crystals of 2-oxazolidinone X-irradiated at 295 K

Abstract

When single crystals of the 5-membered heterocyclic ring structure 2-oxazolidinone (C3O2NH5) are irradiated at room temperature, the major radical formed (R1) decays during a period of a few hours, leaving a broad, unstructured EPR spectrum not amenable to analysis. Cooling the crystals to about 140 K immediately after irradiation at room temperature allows analysis of the R1 radical by EPR and ENDOR. The radical is formed by a net H-abstraction from one of the two methylene groups of the molecule. A full EPR and ENDOR analysis of four proton interactions (one α-coupling, and three β-couplings) together with ESR evidence for a small nitrogen hyperfine interaction allowed for a precise identification of R1. The results show that the carbon-centered radical R1 is puckered at the radical center. Using DFT calculations together with the experimental EPR and ENDOR results, the torsion angle of the C1–H bond with respect to the N–C1–C2 plane is estimated to be 13–15° (bending angle θ ≈ 7°). The DFT calculations reproduced the carbon-bonded proton hyperfine coupling constants satisfactorily but failed to reproduce the experimental results for the nitrogen and nitrogen-bonded proton hyperfine interactions.