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

Gas-phase (CO 2 ) + 2 - 4 cations and (CO 2 ) 2 -, CO 3 -, CO 3 - ·H 2 O and CO 3 - ·CO 2 anions have been examined using density functional theory calculations. It is shown that the lowest energy (CO 2 ) n + cations are C 2 h 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 2 B u and 2 A g states. The “ladder” structure accounts for several trends observed in the photodestruction spectra of the (CO 2 ) n + cations. For the (CO 2 ) 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) CO 2 . A close similarity between the VIS spectra of the (N 2 O) 2 + dimer cation (in sc CO 2 ) and the solvent cation suggests that the latter has the (CO 2 ) 2 + dimer cation as the chromophore core. It is demonstrated that optical, Raman, and magnetic resonance spectra of the carbonate radical anion, CO 3 -, can be consistently accounted for by a C 2 v symmetric structure with the unique O−C−O angle of ≈113° (in water) to ≈100° (in carbonate minerals). The oscillator strength of the A 2 A 1 ← X 2 B 2 transition in the visible correlates with this O−C−O angle. In aqueous CO 3 -, the trigonal distortion is due to hydrogen bonding to a single water molecule. In sc CO 2 (and other nonpolar liquids), the trigonal distortion is weak, and CO 3 - should be a poor light absorber. This may be the reason no dissociative electron capture in sc CO 2 has been observed by optical spectroscopy.