Carbon Dioxide Capture and Release by Anions with Solvent‐Dependent Behaviour: A Theoretical Study

Recently, Clyburne and co‐workers Science, 2014 , 344, 75–78] reported the novel synthesis of the elusive cyanoformate anion, NCCO₂^?. The stability of this anion is dependent on the dielectric constant of the local environment (polarity‐switchable solvent): it is stable in low‐polarity media and unstable in high‐polarity solvents; hence, capturing and then releasing CO₂. The possibility of extending such behaviour to other anions is explored herein. Specifically, the CO₂ capture process is studied for 26 anions in the gas phase and 3 distinct solvents (water, tetrahydrofuran, and toluene) by using the polarisable continuum model. Calculations are performed with the M06‐2X and B3LYP‐D3 density functionals and the aug‐cc‐pVTZ basis set. The design of new CO₂ complexes with the anion, which can be formed or destroyed on demand by changing the solvent, is possible; the results for the alkoxylate and thiolate anions are especially promising. The nature of the substituents connected to the atom that bonds to CO₂ in the anion is crucial in modulating the relative stability of the products—a key point for reversibility in the CO₂ capture process. A moderate interaction for the anion–CO₂ adduct—about 10 kcal mol^?1 relative free energy with respect to the isolated reactants in the gas phase—and a relevant effect in the dielectric constant of the local environment are also key ingredients to achieve solvent dependency.