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

The solvated electron in CH₃CN is scavenged by CO₂ with a rate constant of 3.2 × 10¹⁰ M^-1 s^-1 to produce the carbon dioxide radical anion (CO₂ ^-), 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 CO₂ ^- stretch. This assignment is confirmed by ¹³C isotopic labelling experiments and DFT calculations. In neat CH₃CN, CO₂ ^- decays on a ∼10 μs time scale via recombination with solvent-derived radicals (R) and solvated protons. Upon addition of formate (HCO₂ ^-), the radiation yield of CO₂ ^- is substantially increased due to H-atom abstraction by R from HCO₂ ^- (R + HCO₂ ^- → RH + CO₂ ^-), which occurs in two kinetically separated steps. The rapid step involves the stronger H-abstracting CN, CH₃, and possibly, H primary radicals, while the slower step is due to the less reactive, but more abundant radical, CH₂CN. The removal of solvent radicals by HCO₂ ^- also results in over a hundredfold increase in the CO₂ ^- lifetime. CO₂ ^- scavenging experiments suggest that at 50 mM HCO₂ ^-, about 60% of the solvent-derived radicals are engaged in CO₂ ^- generation. Even under CO₂ saturation, no formation of the radical adduct, (CO₂)₂ ^-, could be detected on the microsecond time scale.