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

Abstract

Abstract 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─CH 2 ─CH 3 ) 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 (C 2 mim) cation‐based C 2 mim 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─CH 2 ─CH 3 ) 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 ν s (CH 2 ) and ν as (CH 3 ) 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 C 4/5 ─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 (C 2 ─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, C 2 mim BF 4 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.