Impact of water on the hydrogen bonding between halide‐based ion‐pairs investigated by Raman scattering and density functional theory calculations

We present a study of the impact of water on the size‐ and electronegativity‐dependent hydrogen bonds between cation and anion of imidazolium‐based ionic liquids (ILs) with halide anions. We have combined Raman spectroscopy with density functional theory (DFT) computations to characterize the intermolecular associations of cation–anion–water moieties. The Raman band at ~1,422 cm−1, which is assigned to the ν(N─CH2─CH3) vibration of the ethyl chain in the neat ion pairs (pure ion pairs), is blue shifted by +7, +4, and +3 cm−1 for 1‐ethyl‐3‐methylimidazolim (C2mim) cation‐based C2mim X ion pairs with X = Cl, Br, and I, respectively, in a water‐rich environment. Simultaneously, upon dilution, a new peak at ~1,438 cm−1 starts to appear at the high‐wavenumber side with continuously increasing intensity. These significant perturbations of the ν(N─CH2─CH3) vibrational mode in water environment are a clear indication of the change in the alkyl chain conformation. Further, the relative intensities of the Raman bands at ~2,946 and ~2,950 cm−1 corresponding to νs (CH2) and νas (CH3) of the ethyl chain, respectively, were significantly changed upon dilution, which also confirms the conformational change of the alkyl chain. The Raman band belonging to the C4/5─H stretch mode was significantly blue shifted by +33, +41, and +42 cm−1 going from the studied neat ion pairs having Cl, Br, and I anions to the corresponding water mixtures, respectively. Significant changes were also observed for the ν(C2─H) vibrational mode of the ion pairs in water environment; here, we observed blue shifts of +60, +51, and +50 cm−1 for ion pairs having Cl, Br, and I anions, respectively. These results confirm the weakening of the ion‐pair interaction due to the interaction with water molecules. In contrast to the ILs with halide anions, C2mim BF4 does not show any variation in the Raman peak positions upon dilution. Experimental results are nicely supported by the results of DFT calculations, and a sensible connection was established. Raman spectra detected the alkyl–chain conformation of size‐ and electronegativity‐dependent halide‐based ionic liquids and their variation in water environment. Ring (C─H) stretching, especially C2─H stretch modes are marker bands for the characterization of the structural changes of halide‐based ionic liquids.