EPR and ENDOR studies of CO2− centers in x- and uv-irradiated single crystals of sodium formate

Abstract

Using ENDOR we have characterized the ²³Na and ¹H hyperfine structure of the CO₂⁻ radical ion in sodium formate single crystals. In contrast to previous studies we have detected CO₂⁻ in uv irradiated samples. ENDOR studies of the ²³Na hfs together with CNDO calculations indicate that the CO₂⁻ forms a tight ion pair with the nearer Na⁺ cation, thus explaining the four line EPR hfs observed. Studies of the temperature and angular dependences of the magnetic resonance parameters suggest that in NaHCO₂ the CO₂⁻ radical anion is held rigidly in the lattice. Hf interactions have been resolved for four pairs of nearest neighbor protons and show considerable covalency in the NaHCO₂ structure, in agreement with recent SCF ab initio calculations on CO₂⁻, which show that the unpaired electron is in a diffuse orbital. Extra so‐called ’’forbidden’’ lines have been observed in the ²³Na ENDOR and a model of ENDOR enhancements involving cross relaxation with other paramagnetic species is discussed.