Electron spin resonance and structure of the CO- 2 radical ion

Analysis of the electron spin resonance spectrum shows that CO^? ₂ radical ions are trapped in sodium formate at room temperature after irradiation of the crystal with high energy γ-rays. This radical is bent with an angle of about 134° and has a ² A ₁ ground state. From the experimental values of the ¹³C coupling coefficients and the g values, coefficients are derived for the mixing of atomic orbitals in the half-filled 4a ₁ molecular orbital which appears to be 14 per cent carbon 2s, 66 per cent carbon 2p _z and 11 per cent of each oxygen 2p _z . There is also coupling to the sodium nucleus in an adjacent cation.