LAUSR

Density functional theory was employed to investigate the interactions between 1-ethyl-3-methylimidazolium ([EMIM]+)/1-ethylpyridinium ([EPY]+)/1-ethyl-1-methylpyrrolidinium ([EPYRO]+)/1-ethyl-1-methylpiperidinium ([EPIP]+) cations and trifluoromethanesulfonate ([OTf]−) anion, as well as the interactions between trifluoromethanesulfonate ([EMIM]+[OTf]−)/1-ethylpyridinium trifluoromethanesulfonate ([EPY]+[OTf]−)/1-ethyl-1-methylpyrrolidinium trifluoromethanesulfonate ([EPYRO]+[OTf]−)/1-ethyl-1-methylpiperidinium trifluoromethanesulfonate ([EPIP]+[OTf]−) and thiophene. The ionic liquids and complexes formed between [EMIM]+[OTf]−/[EPY]+[OTf]−/[EPYRO]+[OTf]−/[EPIP]+[OTf]− and thiophene were analyzed by natural bond orbital, atoms in molecules, and noncovalent interaction, HOMO-LUMO overlap integral analyses, and electron density difference analysis. The calculated results show that thiophene ring is parallel to [EMIM]+/[EPY]+ ring, beneficial to π-π interaction, while [EPYRO]+/[EPIP]+ branched chains tend to form C-H ⋯ π interactions with thiophene. The interactions between [EMIM]+/[EPY]+/[EPYRO]+/[EPIP]+ and thiophene are stronger than that of [OTf]− and thiophene proposed by interaction energy and electron density differences. The HOMO-LUMO overlap integral analyses demonstrate the different charge transfer direction and interacting nature of frontier orbitals.