A theoretical investigation on the adsorption of CO2, N2, O2 and H2 in 1-buty-3-methylimidazolium heptafluorobutyrate ionic liquid

The adsorption of CO2, N2, O2 and H2 in 1-buty-3-methylimidazolium heptafluorobutyrate, [C4mim][CF3CF2CF2COO], was theoretically studied under isolated conditions. It was shown that in gas–[CF3CF2CF2COO]− complexes, the gas molecules are directed toward oxygen atoms of the [CF3CF2CF2COO]− anion, while in gas–[C4mim]+ complexes the gas molecules interact with the imidazolium ring via hydrogen bond formation with C2H. The relative position of the anion and the cation in the ion pair is almost unaffected by the presence of the gas molecules. In order to have a better understanding on the difference in gas solubility, the interaction energies between the gas molecules and the anion, cation and the whole ion pair were calculated. It was found that for gas–[cation] and gas–[ion pair] systems, the order based on the interaction energies estimated at PBE/aug-cc-pVDZ level is qualitatively in agreement with that based on the experimental data, while the same is not true for the gas–[anion] systems.