Energetics of CO2- in Aqueous Solution.

Gas-phase and aqueous solution properties of neutral and anionic clusters of CO2 with 3, 4, and 8 explicit H2O molecules are calculated at the coupled cluster (CCSD(T)) level plus a self-consistent reaction field. Anionic clusters with the radical electron density localized on the carbon of the CO2 molecule rather than localized on the H2O molecules are more favorable energetically by 10-20 kcal/mol in the gas phase (Δ Hgas(298 K)) and 20-30 kcal/mol in aqueous solution (Δ Gaq(298 K)). The most favorable structures are those with the largest number of strong hydrogen bonds between the CO2- and the explicit H2O molecules. Adiabatic electron affinities were calculated in the gas phase and in aqueous solution for the microsolvated anion. The adiabatic electron affinity of aqueous CO2- is predicted to be 2.35 ± 0.08 eV and is converged with as few as 3 explicit H2O molecules plus a self-consistent reaction field. The EA of aqueous CO2 is significantly greater than the aqueous solvation free energy of the electron. The vertical attachment energies to CO2 and the vertical detachment energies from CO2- were calculated. The solvated CO2- anion is substantially bent to 135°, which requires 1.52 eV. The large energy required for bending in combination with the vertical detachment and attachment energies shows that substantial local solvent reorganization occurs on detachment or attachment of an electron to solvated CO2. The formation of aqueous C2O42- from CO2- was also explored, and dimerization is predicted to occur.