Impact of alkyl chain length and water on the structure and properties of 1-alkyl-3-methylimidazolium chloride ionic liquids

Abstract

The influence of the length of the alkyl chain and water molecules on the hydrogen-bond interaction of the chloride anion and imidazolium-based cation of the ionic liquid (IL) C_nmim Cl (where n = 2, 4, 6, 8, and 10) was investigated by combining attenuated total internal reflection infrared (ATR-IR) spectroscopy and density functional theory (DFT) calculations. Here, for the first time, the conformational isomerism of the alkyl chain of C_nmim Cl (n = 2, 4, 6, 8, and 10) is identified by marker IR bands. The IR peak at 1470 cm⁻¹ related to the alkyl chain vibration exhibits a significant perturbation in its intensity and further shows a red shift upon increasing alkyl chain length. This indeed might be a marker IR band for conformational isomerism and also an indication of the interaction of the alkyl chain with the chloride anion. Further, in the C–H vibration region of the IR spectra, a significant variation of the IR intensities was observed for the ν_s(CH₂) and ν_as(CH₂–CH₃) modes at 2931 and 2976 cm⁻¹, respectively. These bands can be considered as further markers for conformational isomerism of the alkyl chain. Moreover, the peak at 2976 cm⁻¹ assigned to an alkyl chain vibration reveals the maximum red shift of 20 cm⁻¹ for n = 10, which suggests charge redistribution among ion-pairs as a result of the alkyl chain variations. Noticeably, the C₂–H vibration does not show any significant change of its wavenumber position, suggesting that the alkyl chain length does not interfere with the hydrogen bond interaction between C₂–H and the Cl anion. This was also evident from the DFT-calculated bond strength between C₂–H and Cl, which remains unchanged upon varying the alkyl chain length. In aqueous solutions, blue shifts of the v(C₂–H) band by +65, +60, +67, +62 and +62 cm⁻¹ for C_nmim Cl (n = 2, 4, 6, 8, and 10) are observed, respectively. These results point to a weakening of the hydrogen bond between cation and anion, which is also supported and validated by results of the solvent (water) effect obtained using the polarized continuum model (PCM) of the DFT calculations.