π-Hole interaction: a theoretical insight into the mechanism of SO2 captured by [Et2NEMim][Tetz] ionic liquids

The mechanism of SO2 capture by 1-(2-diethylaminoethyl)-3-methylimidazolium tetrazolate ([Et2NEMim][Tetz]) was investigated using B3LYP hybrid density functional methods at 6-31 + G(d,p) level. In order to find the origin of the high capacity of the subjected ionic liquids (IL) for SO2 capture, the 1: n (n = 1–5) complexes formed between [Et2NEMim][Tetz] and 1–5 SO2 molecules were optimized. Two interaction modes (π-hole interaction and hydrogen bond) were found in each 1: n (n = 1–5) complex; the second order perturbation stabilization energies, E(2)s, confirmed that the main interaction mode was a π-hole interaction. The calculated interaction energies indicated that the first SO2 absorption should be chemical absorption. The capture of the second and third SO2 should fall between chemical and physical interaction, and the fourth and fifth SO2 are incorporated by physical absorption. Thermodynamic analyses indicated that SO2 capture favors lower temperature and higher pressure. Owing to the interactions between SO2 and the [Tetz] anion or the [Et2NEMim] cation, the SOO asymmetric stretching frequency exhibits an obviously red shift in the complex. The strong absorptions of SOO asymmetric stretching in complex (1:5) appear at 1295 cm−1 (interaction between SO2 and the [Tetz]− anion) and 1247 cm−1 (interaction between SO2 and the tertiary nitrogen on the cation).