Interactions in bisamide ionic liquids—insights from a Hirshfeld surface analysis of their crystalline states

The intermolecular interactions of a series of crystallised bis(trifluoromethanesulfonyl)amide (NTf2) and bis(methanesulfonyl)amide (NMes2) ionic liquids are qualitatively investigated and compared using Hirshfeld surface and thermal analysis techniques. The NMes2 salts are known to exhibit higher glass transitions and higher viscosities than those of the NTf2 salts. The origins of these differences were analysed in terms of the importance of factors such as the C–H⋯O hydrogen bond, fluorination, presence of an aromatic moiety and length of alkyl chain, using the Hirshfeld surfaces and their associated fingerprint plots. Additionally, the existence of C–F⋯π and C–H⋯π interactions were elucidated and the significance of anion–anion interactions was recognised. Overall, these results demonstrate the applicability of the Hirshfeld surface approach in investigating the molecular origins of the physical properties.