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

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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.