Liquid structure of N-butyl-N-methylpyrrolidinium bis-(trifluoromethanesulfonyl) amide ionic liquid studied by large angle X-ray scattering and molecular dynamics simulations

Large angle X-ray scattering of the N-butyl-N-methylpyrrolidinium bis-(trifluoromethanesulfony) amide (P14+TFSA−) ionic liquid was measured at 298 K. The total interference function iLAXS(s) and the total pair correlation function GLAXS(r) were successfully obtained. The iLAXS(s) was analyzed in terms of the intra-molecular geometries of the conformational isomers of the P14+ and TFSA− ions based on the crystal structures, the inter-molecular correlation function GinterLAXS(r) and the inter-molecular radial distribution function as the form DinterLAXS(r) − 4πr2ρ0 were evaluated. With DinterLAXS(r) − 4πr2ρ0, the inter-molecular correlation peaks at 3.5, 4.5 and 5-6 Å evidently appeared as the short range interaction, while considerable long range interactions were found as the broad peaks at 10 and 15 Å. Molecular dynamics simulations of the ionic liquid based on the effective pairwise potentials were carried out. The X-ray total interference function derived from the MD simulations iMD(s) was in agreement with the experimental one. According to the inter-molecular partial atom-atom correlation functions from MD simulations, the observed peaks at 3.5, 4.5 and 5-6 Å can be predominantly ascribed to the closest atom-atom correlations between the P14+ and TFSA- ions such as C···O and C···F. In the inter-molecular partial atom-atom correlation function between terminal methyl carbons of the butyl chain in the P14+ ion, a peak at 4.0 Å was found as the inter-molecular correlation, suggesting that the ionic liquid shows an alkyl chain aggregation, which is similar to that observed for 1-alkyl-3-methylmidazolium ionic liquids.