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.
ASJC Scopus subject areas
- Physics and Astronomy(all)
- Physical and Theoretical Chemistry