ESR Spectrum of O3− Trapped in a Single Crystal of Potassium Chlorate

The O3− radical obtained by x irradiation of oxygen-17 enriched KClO3 was studied by ESR. The molecule is trapped at three distinct sites in the potassium chlorate single crystal, one of which gives rise to a more intense ESR spectrum. For O3− at the dominant trapping site, the principal values of the g tensor are 2.0035, 2.0187, and 2.0123. Two distinct oxygen-17 hyperfine splittings were observed and their variation as a function of orientation studied. The principal values of the corresponding hyperfine tensors are 82.6, −8, −8 and 43.6, −5, −5 G. The direction of the maximum principal value for both oxygen-17 hyperfine tensors coincides with the direction of the minimum principal value of the g tensor, as expected for a bent 19-electron molecule. The higher splitting is assigned to the central atom and the lower splitting to the magnetically equivalent outer atoms. The spin density distribution in O3− deduced from experimental data is compared with theoretical results obtained from a CNDO/II calculation. The g values for O3− at the less populated sites are practically identical to the g values given above. Oxygen-17 hyperfine splittings observed from O3− at these sites seem to indicate a slight distortion of the molecule, possibly from C2v to Cs symmetrv. All magnetic parameters were measured at 77°K.