ESR of 17O-Labeled CO2− Radical Trapped in a Single Crystal of Sodium Formate

The ESR spectrum of x‐irradiated sodium formate enriched in ¹⁷O is interpreted in terms of the CO₂⁻ radical. Two magnetically inequivalent oxygens were observed. The hyperfine tensors of ¹³C and ¹⁷O were determined and the following principal values were obtained for ¹³C: 155, 150, and 195 G, respectively, along the axis perpendicular to the molecular plane, along the O–O direction, and along the $C_2$ axis. These results agree with those previously reported. For ¹⁷O the magnitudes of the principal values for both oxygens are practically identical, the averages being 22.0, 23.2, and 51.0 G. The direction of the largest value is in the molecular plane inclined at 13° to the $C_2$ axis. The results are used to derive the coefficients of the ${\text{4a}}_1$ molecular orbital containing the unpaired electron.