Radiolytic formation of the carbon dioxide radical anion in acetonitrile revealed by transient IR spectroscopy

The solvated electron in CH 3 CN is scavenged by CO 2 with a rate constant of 3.2 × 10 10 M −1 s −1 to produce the carbon dioxide radical anion (CO 2 &z.rad; − ), a strong and versatile reductant. Using pulse radiolysis with time-resolved IR detection, this radical is unambiguously identified by its absorption band at 1650 cm −1 corresponding to the antisymmetric CO 2 &z.rad; − stretch. This assignment is confirmed by 13 C isotopic labelling experiments and DFT calculations. In neat CH 3 CN, CO 2 &z.rad; − decays on a ∼10 μs time scale via recombination with solvent-derived radicals (R&z.rad;) and solvated protons. Upon addition of formate (HCO 2 − ), the radiation yield of CO 2 &z.rad; − is substantially increased due to H-atom abstraction by R&z.rad; from HCO 2 − (R&z.rad; + HCO 2 − → RH + CO 2 &z.rad; − ), which occurs in two kinetically separated steps. The rapid step involves the stronger H-abstracting CN&z.rad;, CH 3 &z.rad;, and possibly, H&z.rad; primary radicals, while the slower step is due to the less reactive, but more abundant radical, CH 2 CN&z.rad;. The removal of solvent radicals by HCO 2 − also results in over a hundredfold increase in the CO 2 &z.rad; − lifetime. CO 2 &z.rad; − scavenging experiments suggest that at 50 mM HCO 2 − , about 60% of the solvent-derived radicals are engaged in CO 2 &z.rad; − generation. Even under CO 2 saturation, no formation of the radical adduct, (CO 2 ) 2 &z.rad; − , could be detected on the microsecond time scale. First IR detection of CO 2 &z.rad; − in acetonitrile, produced by radiation-induced CO 2 reduction and oxidation of formate.