The nature of the CO2 (-) radical anion in water.

Abstract

The reductive conversion of CO2 into industrial products (e.g., oxalic acid, formic acid, methanol) can occur via aqueous CO2 (-) as a transient intermediate. While the formation, structure, and reaction pathways of this radical anion have been modelled for decades using various spectroscopic and theoretical approaches, we present here, for the first time, a vibrational spectroscopic investigation in liquid water, using pulse radiolysis time-resolved resonance Raman spectroscopy for its preparation and observation. Excitation of the radical in resonance with its 235 nm absorption displays a transient Raman band at 1298 cm(-1), attributed to the symmetric CO stretch, which is at ∼45 cm(-1) higher frequency than in inert matrices. Isotopic substitution at C ((13)CO2 (-)) shifts the frequency downwards by 22 cm(-1), which confirms its origin and the assignment. A Raman band of moderate intensity compared to the stronger 1298 cm(-1) band also appears at 742 cm(-1) and is assignable to the OCO bending mode. A reasonable resonance enhancement of this mode is possible only in a bent CO2 (-)(C2v/Cs) geometry. These resonance Raman features suggest a strong solute-solvent interaction, the water molecules acting as constituents of the radical structure, rather than exerting a minor solvent perturbation. However, there is no evidence of the non-equivalence (Cs) of the two CO bonds. A surprising resonance Raman feature is the lack of overtones of the symmetric CO stretch, which we interpret due to the detachment of the electron from the CO2 (-) moiety towards the solvation shell. Electron detachment occurs at the energies of 0.28 ± 0.03 eV or higher with respect to the zero point energy of the ground electronic state. The issue of acid-base equilibrium of the radical, which has been in contention for decades, as reflected in a wide variation in the reported pKa (-0.2 to 3.9), has been resolved. A value of 3.4 ± 0.2 measured in this work is consistent with the vibrational properties, bond structure, and charge distribution in aqueous CO2 (-).