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Title: The nature of the CO2- radical anion in water

Journal Article · · Journal of Chemical Physics
DOI:https://doi.org/10.1063/1.4946868· OSTI ID:1470757
ORCiD logo [1];  [1]
  1. Univ. of Notre Dame, IN (United States). Radiation Lab.
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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, here, for the first time, we present 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 (13CO2-) 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-.

Research Organization:
Univ. of Notre Dame, IN (United States)
Sponsoring Organization:
USDOE Office of Science (SC), Basic Energy Sciences (BES). Chemical Sciences, Geosciences, and Biosciences Division
Grant/Contract Number:
FC02-04ER15533
OSTI ID:
1470757
Alternate ID(s):
OSTI ID: 1248059
Journal Information:
Journal of Chemical Physics, Vol. 144, Issue 15; ISSN 0021-9606
Publisher:
American Institute of Physics (AIP)Copyright Statement
Country of Publication:
United States
Language:
English
Citation Metrics:
Cited by: 48 works
Citation information provided by
Web of Science

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Cited By (8)

Infrared Spectroscopy of Size‐Selected Hydrated Carbon Dioxide Radical Anions CO 2 .− (H 2 O) n ( n =2–61) in the C−O Stretch Region journal June 2019
Infrared Multiple Photon Dissociation Spectroscopy of Hydrated Cobalt Anions Doped with Carbon Dioxide CoCO 2 (H 2 O) n − , n =1–10, in the C−O Stretch Region journal January 2020
Photocatalytic Reverse Semi‐Combustion Driven by Ionic Liquids journal February 2019
Single- and double-electron reductions of CO 2 by using superalkalis: An ab initio study journal February 2018
Photochemistry of glyoxylate embedded in sodium chloride clusters, a laboratory model for tropospheric sea-salt aerosols journal January 2018
Radiolytic formation of the carbon dioxide radical anion in acetonitrile revealed by transient IR spectroscopy journal January 2018
The selenocyanate dimer radical anion in water: Transient Raman spectra, structure, and reaction dynamics journal March 2019
On the origin of the elusive first intermediate of CO 2 electroreduction journal September 2018

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38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEAR CHEMISTRY