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Issue 15, 2018
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Radiolytic formation of the carbon dioxide radical anion in acetonitrile revealed by transient IR spectroscopy

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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.

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

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Publication details

The article was received on 10 Feb 2018, accepted on 29 Mar 2018 and first published on 29 Mar 2018


Article type: Paper
DOI: 10.1039/C8CP00977E
Citation: Phys. Chem. Chem. Phys., 2018,20, 10011-10017
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    Radiolytic formation of the carbon dioxide radical anion in acetonitrile revealed by transient IR spectroscopy

    D. C. Grills and S. V. Lymar, Phys. Chem. Chem. Phys., 2018, 20, 10011
    DOI: 10.1039/C8CP00977E

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