Time-resolved resonance Raman spectroscopy of the carbonate radical

Abstract

The literature contains conflicting evidence regarding the protonation state of the carbonate radical, a species which now appears to be of biological significance. The time-resolved resonance Raman spectrum of CO₃^- has been observed. The radical was produced by oxidation of bicarbonate and carbonate using sulfate radicals at pH values from 7.5 to 12.3. The resonance Raman spectrum was found to be invariant with pH and contains a strongly polarised and intense band at 1062 cm^-1. The spectrum is consistent with the radical having C_2v symmetry, indicating some distortion from the predicted D_3h structure. The data suggest that the carbonate radical (CO₃^-) formed by one-electron oxidation of bicarbonate and carbonate does not undergo protonation to the conjugate acid, HCO₃, as previously suggested with a pK_a of 9.6. Some biochemical consequences of this are discussed.