Evidence of C-F-P and aromatic pi-F-P weak interactions in imidazolium ionic liquids and its consequences

Abstract

A simple change from alkyl group to alkene in side chain of imidazolium cation with same anion resulted in a drastic impact on physical properties (e.g., melting point) from bmimPF(6) IL to cmimPF(6) IL The underlying reasons have been elucidated by structural and interaction studies with the help of DSC, SCXRD, vibrational and multi-nuclear NMR spectroscopic techniques. Experiments reveal existence of new weak interactions involving the carbon and pi cloud of the imidazolium aromatic ring with fluoride of PF6 anion (i.e., C-2-F-P and pi-F-P) in cmimPF(6) but are absent in structurally similar prototype IL bmimPF(6). Though weak, these interactions helped to form ladder type supramolecular arrangement, resulting in quite high melting point for cmimPF(6) IL compared to bmimPF(6) IL These findings emphasize that an IL system can behave uniquely because of the existence of uncommon weak interactions. (C) 2017 Elsevier B.V. All rights reserved.