Comparative study between 1-Propyl-3-methylimidazolium bromide and trimethylene bis-methylimidazolium bromide ionic liquids by FTIR/ATR and FT-RAMAN spectroscopies

Abstract : In this study, we synthesized two ionic liquids based on imidazolium: one is a monocationic and the other is a dicationic. They are respectively 1-Propyl-3-methylimidazolium bromide ([PrMIM+][Br−]) and trimethylene bis-methylimidazolium bromide ([M(CH2)3IM2+][2Br−]). The structures of these two ionic liquids which are composed of ions with atoms of the same nature were first identified by 1H,13C NMR, and then compared in a study by FT-RAMAN and FTIR/ATR spectroscopies. FT-RAMAN spectras of the dicationic ionic liquid are richer in modes in the different spectral regions. Hence this richness seems to be a consequence of the passage from one to two rings in the imidazolium cation. In particular, the vibrational modes in the spectral ranges 700–600 cm−1, 1700–1500 cm−1 and 3200–2700 cm−1 by FTIR/ATR seem to be sensitive to the change from mono to dicationic than in FT-RAMAN. The spectral range in which the intermolecular interactions are present (200-50 cm−1) is a marker of differentiation between the mono and the dicationic. The spectral ranges on 1700–1200 cm−1 and 3200–2700 cm−1 also show signs of upheaval between our two samples. We can also notice that there are much more active modes in FT-RAMAN spectroscopy than in FTIR/ATR spectroscopy. © 2017 Elsevier B.V.