MP2 versus density functional theory calculations in CO2-sequestration reactions with anions: Basis set extrapolation and solvent effects

The emission of carbon dioxide in large amounts is commonly believed to be the main cause of global climate changes. Development of CO2 capture processes is still a big current challenge. Some anions have been studied for the gas sequestration process due their great affinity to CO2. In this work, electronic structure calculations were performed at the MP2/aug-cc-pvtz level to compute the interaction between 20 anions and CO2. A complete basis set scheme, using extrapolated energies, was also employed for both gas phase and solvent calculations. The reactions between the anions and CO2 were therefore studied in four different conditions (gas phase, toluene, tetrahydrofuran and water). The trends observed for the reaction thermodynamics with the MP2 method is similar to that observed previously with the B3LYP-D3 and M06-2X functionals. The reactions in the gas phase are highly exothermic and do not involve any activation energy. The solvent effect reduces the exothermicity and induces an intrinsic activation barrier. The negative charge is dispersed in the adduct, leading to a weaker interaction in a polar solvent. Then, increasing the medium polarity, the energy difference between the adduct and the reactants decreases. We also observed a limit for solvent stabilization in the low dielectric constant range. For example, the results obtained with tetrahydrofuran are closer to those obtained with water than to those obtained with toluene. Considering both the thermodynamics of the reaction and the differential solvent effects, we were able to indicate anions derived from alkyl sulfides as the most convenient for CO2 sequestration among the set here considered.