Conclusion
The reactions of CO2 with 20 anions were computed with the wavefunction MP2/aug-cc-pvtz method. To test for convergence of the results, we also employed an extrapolation procedure using the CBS approach, as reported in previous studies. Solvent effects were computed using the IEFPCM method with water, tetrahydrofuran and toluene as solvents. In the gas-phase the computed reactions are exergonic without any activation energy. In the condensed phase the anions are more stabilized than the adducts, reducing the reaction spontaneity and inducing an activation barrier for some systems. Interestingly, although the dielectric constant of tetrahydrofuran is closer to that of toluene than to the dielectric constant of water, results obtained with tetrahydrofuran are closer to results obtained with water than with toluene. For some anions we observe a change in the reaction spontaneity in the different solvents, allowing to think of a medium where CO2 sequestration and regeneration could be done depending only on the medium composition. Alkyl sulfide anions emerge as an interesting candidate for CO2 sequestration because they combine suitable thermodynamics with low activation barrier for both the direct and the reverse reactions.
When comparing the trends obtained with the different methods, we observe that the MP2 and CBS approaches yield results that are close to each other within 3 kcal mol-1. Comparing to results obtained previously with the B3LYP-D3 and M06-2X functionals we found that B3LYP-D3 follows the same trends as MP2, while M06-2X tends to diverge from the wavefunction results.