Density functional study of the mechanism of nitric oxide captured by [Emim][OTf] ionic liquids

A detailed understanding of the mechanism of nitrogen oxides (NOx) absorption by ionic liquids (ILs) is of great significance for the application of ILs in the gas capture and separation process. The interaction energies between the cation ([Emim] +) or the anion ([OTf] -) of 1-ethyl-3-methylimidazolium trifluoromethanesulfonate ILs and NO2 (or N2O4) are estimated at the M06-2X/6-31+G** level with basis set superposition error (BSSE) correction. Two interaction models in the complexes of cation with NO2 (or N2O4) have been considered. The pi(cation)-pi(NO2) mode is favorable than the hydrogen-bonding model while there is a preference for the hydrogen-bonding model to the pi(cation)-pi(N2O4) in the cation- N2O4 system. The calculated results demonstrate that the interaction between the anion and NO2 (or N2O4) is the dominant factor in determining the absorption process of NO2 by the ILs and the expected pi(cation)-pi stacking contribution to the efficient absorption of NO2 is not reproduced. This may indicate that the role of the pi(cation)-pi N2O4 stacking interaction may have been overestimated. The NBO and AIM analysis further provide evidence for the interaction model and strength between the ion and the gas molecules.