On the interactions between 1-butyl-3-methylimidazolium hydrosulphate and indole/quinoline

Abstract Context The removal of nitrogen-containing compounds from fossil fuels is crucial for improving fuel stability and reducing NOₓ emissions, while conventional hydrodenitrogenation is limited by high energy consumption and catalyst deactivation. Ionic liquid–based extraction offers a promising alternative, particularly for refractory nitrogen species. In this study, the molecular interactions between the sulfate-based ionic liquid 1-butyl-3-methylimidazolium hydrosulfate ([BMIM][HSO 4 ]) and two representative nitrogen-containing compounds—indole (neutral) and quinoline (basic)—were investigated using density functional theory. The results show that [BMIM][HSO 4 ] interacts more strongly with indole than with quinoline, primarily due to the formation of a strong N–H···O hydrogen bond between indole and the [HSO 4 ]⁻ anion. Both the cation and anion contribute cooperatively through hydrogen bonding, electrostatic interactions, and π···π stacking. These findings provide a molecular-level explanation for the high extraction efficiency of sulfate-based ionic liquids toward indole and offer guidance for the rational design of task-specific ionic liquids for extractive denitrogenation. Methods All calculations were carried out using density functional theory at the ωB97XD/6-311 + + G(d,p) level. Full geometry optimizations were performed without symmetry constraints, and all optimized structures were confirmed as true minima by harmonic frequency analysis. Interaction energies were calculated with correction for basis set superposition error. Natural bond orbital (NBO) analysis was employed to evaluate charge distribution and donor–acceptor interactions, while noncovalent interaction (NCI) and atoms-in-molecules (AIM) analyses were conducted to characterize hydrogen bonding, dispersion, and π···π interactions. Wavefunction analyses, including electrostatic potential mapping, NCI visualization, and AIM topological analysis, were performed using the Multiwfn program.