Electronic and topological properties of interactions between imidazolium-based ionic liquids and thiophenic compounds: a theoretical investigation

To deepen the understanding the interactions of thiophenic compounds in ionic liquids, we have performed a systemic study on the electronic structures, and topological properties of interactions between N-ethyl-N-ethylimidazolium diethyl phosphate (EEIM]DEP]) ionic liquid and 3-methylthiophene (3-MT), benzothiophene (BT), or dibenzothiophene (DBT) using density functional theory. From NBO atomic charges and electrostatic potential analyses, most of the positive charge is located on C2–H2 in the EEIM] cation, and the negative charge is focused on oxygen atoms in DEP] anion, implying oxygen atoms in DEP] should easily attack C2–H2 in EEIM]. The electrostatic interaction between anion and cation may be dominant for the formation of the EEIM]–DEP] ion pair. The large stabilizing effect is due to the strong orbital interactions between the antibonding orbital of proton donor σ*(C2–H2) in EEIM] cation and the lone pairs of proton acceptor LP(O) in DEP] anion. A common feature of EEIM]DEP], EEIM]DEP]-3-MT/BT/DBT complexes is the presence of hydrogen bonds between EEIM] cation and DEP] anion. This work has also given the interacting mechanism of 3-MT, BT, and DBT adsorption on EEIM]DEP] ionic liquid. Both EEIM] cation and DEP] anion are shown to play important roles in interactions between 3-MT, BT, DBT and EEIM]DEP], which has been corroborated by NBO and AIM analyses. The π···π, π···C–H and hydrogen bonding interactions occur between EEIM]DEP] and 3-MT, BT, DBT. The strength of sulfur involved interactions between 3-MT, BT, DBT and EEIM]DEP] follows the order of 3-MT > BT > DBT. The order of interaction energies between EEIM]DEP] and 3-MT, BT, DBT is 3-MT < BT < DBT, in agreement with the order of extractive selectivity from fuel oils (DBT > BT > 3-MT) in terms of sulfur partition coefficients.