Impact of Alkyl Chain Length and Water on the Structure and Properties of 1-alkyl-3-methylimidazolium Chloride Ionic Liquids

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) Cnmim Cl (where n = 2, 4, 6, 8, and 10) were 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 Cnmim 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 their intensity and further shows the red shift upon increasing alkyl chain length. This indeed might be a marker IR band for conformational isomerism and also an indication for 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 νs(CH2) and νas(CH2-CH3) modes at 2931 and 2976 cm-1, respectively. These bands can be considered 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 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 the variation of the alkyl chain length. In aqueous solution, blue shifts of the ν(C2-H) band by +65, +60, +67, +62 and +62 cm -1 for Cnmim 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 supported and validated also for the solvent (water) effect using the polarized continuum model (PCM) of the DFT calculation.