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

Abstract

Density functional theory is employed to study the interaction energies between dibenzothiophene (DBT) and 1-alkyl-3-methylimidazolium tetrafluoroborate ([C_nmim]⁺[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 [C_nmim]⁺[BF₄]⁻ has four hydrogen bonds between [C_nmim]+ 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 [C_nmim]⁺[BF₄]⁻.