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

Abstract

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.