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 (C 3 O 2 NH 5 ) 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 a-coupling, and three b-couplings) together with ESR evidence for a small nitrogen hyperfine interaction allowed for a precise identification of R1. The results show that the carboncentered radical R1 is puckered at the radical center. Using DFT calculations together with the experimental EPR and ENDOR results, the torsion angle of the C 1 -H bond with respect to the N-C 1 -C 2 plane is estimated to be 13-15 (bending angle y % 7 ). The DFT calculations reproduced the carbon-bonded proton hyperfine coupling constants satisfactorily but failed to reproduce the experimental results for the nitrogen and nitrogenbonded proton hyperfine interactions.

Alanine Radicals. 2. The Composite Polycrystalline Alanine EPR Spectrum Studied by ENDOR, Thermal Annealing, and Spectrum Simulations

“…The parameters used for the R3 simulation are, with one exception, in good agreement with those calculated using DFT methods. ,, The one exception is the nitrogen-hyperfine interaction. Although DFT methods predict a very anisotropic nitrogen interaction, this is not in agreement with our analysis of the R3 resonance, nor with experimental data for several similar radicals with an N−H fragment in α-position to a carbon with large 2π-spin density for which similar discrepancies were found. − …”

“…The ENDOR spectrum in Figure suggests that the conformation exhibiting the largest methyl hyperfine coupling is dominating after warming and in the following text only this conformation of R3 is taken into consideration. Additional parameters for R3 were constructed on the basis of previously published parameters from similar radicals. − These parameters are listed in Table . Figure c shows the calculated spectrum obtained using these parameters, normalized to unit area.…”

“…Although DFT methods predict a very anisotropic nitrogen interaction, this is not in agreement with our analysis of the R3 resonance, nor with experimental data for several similar radicals with an N-H fragment in R-position to a carbon with large 2π-spin density for which similar discrepancies were found. [25][26][27] The R3 radical is a significant contributor to the EPR spectra obtained after annealing at 480 K. Thus, the collected data indicate that the three radicals in alanine have a very different stability at elevated temperatures. Preliminary measurements have indicated that R1 is reduced by a factor of about 80, R2 by a factor of about 8, and R3 by a factor about 2, comparing spectra obtained immediately after irradiation at room temperature with those obtained after warming at 480 K for 40 min, measured at room temperature.…”

“…Additional parameters for R3 were constructed on the basis of previously published parameters from similar radicals. [25][26][27] These parameters are listed in Table 3. Figure 7c shows the calculated spectrum obtained using these parameters, normalized to unit area.…”

Allyl type radical formation in X-irradiated glutarimide crystals studied by ENDOR and ENDOR-induced EPR

“…Radical III, most probably an allylic type radical, provides indirect evidence that radiation induced reactions involving hydrogen elimination can take place in solid phase glutarimide. These results suggest strongly that further studies should be made of glutarimide and also other simple cyclic imides (e.g., succinimide, oxazolidinone 24 ) in the solid state to elucidate the pathways for secondary radical reactions in these systems.…”