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 (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 C-2h 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 B-2(u) and (2)A(g) 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 C-2nu symmetric structure with the unique O-C-O angle of approximate to113degrees (in water) to approximate to 100degrees (in carbonate minerals). The oscillator strength of the A (2)A(1) <-- X B-2(2) 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.