Hydrogen-bonding interactions between a pyridinium-based ionic liquid [C4Py][SCN] and dimethyl sulfoxide

In this work, the interactions between a representative pyridinium-based ionic liquid (IL) with cyano-functionalized anion ([C4Py][SCN]) and dimethyl sulfoxide (DMSO) were investigated in detail using attenuated total reflection infrared spectroscopy (ATR-FTIR), hydrogen nuclear magnetic resonance (1H NMR), and density functional theory (DFT) calculations. It was found that H-bonds are universally involved and play important role for the miscibility of DMSO with [C4Py][SCN] IL and maintain the stability of this system. ATR-FTIR and excess spectroscopy analysis indicated that the H-bonds involving the alkyl C-Hs and C≡N are strengthened with the addition of DMSO, while the H-bonds involving pyridinium ring C-Hs as well as the H-bonds formed between the [C4Py]+ and [SCN]− are weakened. The addition of [C4Py][SCN] IL led to the H-bonds involving the C-Hs in DMSO weakened comparing with the associated H-bonds in the pure DMSO system. The results of DFT calculations indicated that the DMSO molecules cannot disrupt the strong Coulombic interaction between the [C4Py]+ and [SCN]−. Natural bond orbital (NBO) analysis further confirmed that the interaction mechanisms of DMSO molecule with the anion and cation are different in nature. These studies will shed light on exploring the applications of ILs as reaction or separation media.