Ionic Species in Pulse Radiolysis of Supercritical Carbon Dioxide. 2. Ab Initio Studies on the Structure and Optical Properties of (CO2)n+, (CO2)2-, and CO3- Ions

Abstract

Gas-phase (CO2)+2-4 cations and (CO2)2-, CO3-, CO3-·H2O and CO3-·CO2 anions have been examined using density functional theory calculations. It is shown that the lowest energy (CO2)n+ cations are C2h symmetric “ladder” structures with complete delocalization of charge and spin between the monomer units. These cations absorb in the visible due to a charge resonance band that involves their lowest 2Bu and 2Ag states. The “ladder” structure accounts for several trends observed in the photodestruction spectra of the (CO2)n+ cations. For the (CO2)2+ dimer cation, the energy and the oscillator strength of the charge resonance band compare favorably with these parameters for the solvent radical cation in supercritical (sc) CO2. A close similarity between the VIS spectra of the (N2O)2+ dimer cation (in sc CO2) and the solvent cation suggests that the latter has the (CO2)2+ dimer cation as the chromophore core. It is demonstrated that optical, Raman, and magnetic resonance spectra of the carbonate radical anion, CO3-, can be consistently accounted for by a C2v symmetric structure with the unique O−C−O angle of ≈113° (in water) to ≈100° (in carbonate minerals). The oscillator strength of the A 2A1 ← X 2B2 transition in the visible correlates with this O−C−O angle. In aqueous CO3-, the trigonal distortion is due to hydrogen bonding to a single water molecule. In sc CO2 (and other nonpolar liquids), the trigonal distortion is weak, and CO3- should be a poor light absorber. This may be the reason no dissociative electron capture in sc CO2 has been observed by optical spectroscopy.