Vibrational spectroscopic study of ionic liquids: Comparison between monocationic and dicationic imidazolium ionic liquids

• Synthesis of dicationic ionic liquid [M(CH2)IM2+][2NTf2-].
• FTIR/ATR and FT-RAMAN spectroscopy study of the previous dicationic ionic liquid.
• Comparison between our dicationic ionic liquid and a monocationic ionic liquid.
• There are more active modes in FT-Raman than in FTIR/ATR spectroscopy.
• The passage from mono to dicationic is responsible of important vibrational changes.

In this study, we synthesised a dicationic ionic liquids labeled bis-methyl imidazolium methylidene bis (trifluoromethanesulfonyl) imide ([M(CH2)IM2+][2NTf2-]). The structure was identified by NMR. In order to characterise this ionic liquid, vibrational spectroscopy studies were performed by FTIR/ATR and FT-Raman spectroscopies. A comparative study was introduced by FTIR/ATR and FT-Raman spectroscopies, between our synthesised dicationic ionic liquid and a monocationic ionic liquid ([EMIM+][NTf2-]), where the anion and the cation are similar.FTIR/ATR spectra of dicationic ionic liquid are richer in modes especially in the spectral range of 3000 cm−1. In the 1400–900 cm−1 spectral region, the mode corresponding to the vibration of the NTf2- anion at 1040, 1140 and 1340 cm−1 is very sensitive by the number of cation change. It was also the case for the peak at 1575 cm−1 assigned to ring in-plane symmetric/anti-symmetric stretch CH2(N) and CH3(N)CN stretch. In the 4000–2800 cm−1 spectral region, we observed few new peaks for [M(CH2)IM2+] comparatively with [EMIM+]; a splitting for the peak at 3105 cm−1 and some wavenumber shifts for various modes.For the FT-Raman spectroscopy study, we realised our study in the spectral region 4000–0 cm−1 while that of the monocationique sample is realised in the spectral region 1700–200 cm−1. We observe in the common region 19 new peaks with the passage from the mono to dicationic, what is consequent. The impact of the passage from mono to dicationic is more important for the anion contribution spectral range (1400–300 cm−1) than in the FTIR/ATR spectroscopy study. Other markers of differentiation between [M(CH2)IM2+] and [EMIM+] are observed for vibrational modes assigned to CH2(N) and CH3(N)CN stretch. We can also notice a contrast between the two spectroscopies: there are more active modes in FT-Raman than in FTIR/ATR spectroscopy.

Peaks which are surrounded appears just in the case of the dicationic ionic liquid in FT-RAMAN spectrum.Figure optionsDownload as PowerPoint slide