Estimation of conformational entropy for ionic liquids from NMR spectroscopy

Conformational entropy (Sconf) plays a key role in the low melting points of ionic liquids. In this study, a nuclear magnetic resonance (NMR)-based approach was developed to experimentally estimate Sconf of 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide. J-coupling constants from the 1D 1H NMR and 2D spectra of the butyl group in the cation were analyzed using the Karplus equation optimized by density functional theory calculations. The obtained conformational population and entropy values coincided reasonably well with previously reported results, indicating the usefulness of the NMR approach.