Understanding the role of hydrogen bonding in ether-functionalized ionic liquids by vibrational spectroscopy
Beatriz ROCHA DE MORAES1, Rômulo ANDO1
1University of São Paulo, São Paulo, Brazil
Specific properties and applications of ionic liquids (ILs) can be achieved by the judicious choice of the cation and anion. Among the main modifications in the structure that induce a change in their physicochemical properties is the functionalization of the alkyl chain of the cations. The introduction of ether groups in the side chain leads to important changes in properties including lower melting point and lower viscosity. To understand how the insertion of ether groups induces changes in the physical properties of these systems, in this work imidazolium ionic liquids containing different cations, 1-(2-methoxyethyl)-3-methylimidazolium (C1OC2MIM) and 1-(2-(methoxymethoxy)ethyl)-3-methyl-imidazolium (C1OC1OC2MIM), combined to bromide and acetate (OAc) anions were studied by Raman and infrared spectroscopic techniques. Through the far infrared spectrum (FIR), it’s possible to obtain information about the interionic vibrations, where the contributions below 150 cm-1 for imidazolium ionic liquids can be assigned to modes with major contributions of the bending and stretching vibrational modes associated to the H-bonding between the anion and the H-(C2) of the imidazolium ring. Figure (a) shows the effect of the anion, where the band shift towards higher frequency, ca. 40 cm-1, indicates a stronger interaction of the OAc relative to Br, via hydrogen bonding with the H-(C2). This finding is supported by the DFT calculations of the ionic pair which shows up to 50 kJ mol−1 for [C1OC2MIM][CH3COO] compared to the bromide species. Figure (b) shows the effect of the ether chain in the FIR spectra of the acetate ILs. It is observed a small shift, ca. 5 cm-1 of the maximum, and a subtle broadening of the band. It is well known that the insertion of oxygens atoms in the carbon chains increases the interactions between cations, which can explain the shift to lower wavenumbers and the broadening of the band. In fact, DFT calculations considering an ionic pair or clusters containing four ionic pairs confirm the greater stability with the formation of intra and intermolecular hydrogen bonding of the oxygen atoms and the H-(C2), respectively. For the acetates, the consequence of the larger number of hydrogen bonds performed by the doubly functionalized cation is translated into its physical properties, as higher viscosity, and glass transition lower than 200 K in comparison to C1OC2MIM. The focus of our work is to correlate vibrational spectroscopic data with interionic interactions and how it defines some of the important ionic liquids’ properties.