Synthesis, NMR, Raman, thermal and nonlinear optical properties of dicationic ionic liquids from experimental and theoretical studies

Abstract : In this work, two new di-cationic imidazolium based ILs containing chloride, (bisImCl) and hexafluorophosphate anion (bisImPF 6) have been synthesized and characterized by using FT-Raman and 1 Hand 13 C-NMR spectroscopies. The p-xylyl group acts a spacer group between two imidazolium cations. B3LYP/6-311++G** calculations have optimized the structure of bisIm cation with both anion Cl in monodentate coordinations while in bisImPF 6 only a PF 6 group presents monodentate coordination. AIM calculations at the same level of theory have evidenced a higher number of interactions in bisImPF 6 , as compared with bisImCl. The effect of anions on thermal properties has been investigated. The thermal behavior confirmed that bisImCl and bisImPF 6 have shown good thermal stability. The complete assignments for cation, bisImCl and bisImPF 6 are here reported together with a set of harmonic scaled force constants calculated for cation and bisImCl. Intermolecular and cation-anion interactions have been investigated by NMR spectroscopy. Further, polarizability (α) and the static first hyperpolarizabilities (β) of these ILs are calculated by DFT functionals in order to understand nonlinear optical (NLO) properties. It is also observed an inversed correlation between the predicted β value and the HOMO-LUMO energy difference (Δε) from two state models. The high gap value observed for bisImPF 6 justify its low reactivity, as supported by high number interactions predicted for this ionic liquid.