Theoretical study of interactions between 1-alkyl-3-methyimidazolium tetrafluoroborate and dibenzothiophene: DFT, NBO, and AIM analysis

Density functional theory is employed to study the interaction energies between dibenzothiophene (DBT) and 1-alkyl-3-methylimidazolium tetrafluoroborate ([Cnmim]+[BF4]−). The structures of DBT, 1-ethyl-3-methylimidazolium tetrafluoroborate ([C2mim]+[BF4]−), 1-butyl-3-methylimidazolium tetrafluoroborate ([C4mim]+[BF4]−), 1-hexyl-3-methylimidazolium tetrafluoroborate ([C6mim]+[BF4]−), 1-octyl-3-methylimidazolium tetrafluoroborate ([C8mim]+[BF4]−), [C2mim]+[BF4]−–DBT, [C4mim]+[BF4]−–DBT, [C6mim]+[BF4]−–DBT and [C8mim]+[BF4]−–DBT systems are optimized systematically at the B3LYP/6-31G(d,p) level, and the most stable geometries are obtained by NBO and AIM analyses. The results indicate that DBT and imidazolium rings of ionic liquids are parallel to each other. It is found that the [BF4]− anion prefers to be located close to a C1–H9 proton ring in the vicinity of the imidazolium ring and the most stable gas-phase structure of [Cnmim]+[BF4]− has four hydrogen bonds between [Cnmim]+ and [BF4]−. There are hydrogen bonding interactions, π–π and C–H–π interactions between [C8mim]+[BF4]− and DBT, which is confirmed by NBO and AIM analyses. The calculated interaction energies for the studied ionic liquids can be used to interpret a better extracting ability of [C8mim]+[BF4]− to remove DBT, due to stronger interactions between [C8mim]+[BF4]− and DBT, in agreement with the experimental results of dibenzothiophene extraction by [Cnmim]+[BF4]−.