Vibrational assignments, conformational analysis, and molecular structures of $$\left[ {\text{C}_{\text{n}} \text{mim}} \right]\left[ {\text{NTF}_{\text{2}} } \right]$$ C n mim NTF 2 (n = 2, 4, 6, and 8) imidazolium-based ionic liquids: a combined experimental and quantum chemical approach

Abstract

This work is aimed at providing physical insights about the interactions of cations, anion, and ion pairs of four imidazolium-based ionic liquids of $$\left[ {{\text{C}}_{\text{n}} {\text{mim}}} \right]\left[ {{\text{NTF}}_{2} } \right]$$ with varying alkyl chain lengths (n = 2, 4, 6, and 8) using both DFT calculations and vibrational spectroscopic measurements (IR absorption and Raman scattering) in the mid- and far regions. The calculated Mulliken charge distributions of $$\left[ {{\text{C}}_{\text{n}} {\text{mim}}} \right]\left[ {{\text{NTF}}_{2} } \right]$$ ion pairs indicate that hydrogen-bonding interactions between oxygen and nitrogen atoms (more negative charge) on $$\left[ {{\text{NTF}}_{2} } \right]^{ - }$$ anion and the hydrogen atoms (more positive charge) on the imidazolium ring play a dominating role in the formation of ion pair. Thirteen stable conformers of $$\left[ {{\text{C}}_{2} {\text{mim}}} \right]\left[ {{\text{NTF}}_{2} } \right]$$ were optimized. According to our results, the strongest and weakest hydrogen bonds were existing in $$\left[ {{\text{C}}_{2} {\text{mim}}} \right]\left[ {{\text{NTF}}_{2} } \right]$$ and $$\left[ {{\text{C}}_{8} {\text{mim}}} \right]\left[ {{\text{NTF}}_{2} } \right]$$ , respectively. A redshift of 290, 262, 258, and 257 cm−1 has been observed for cations involving $$\left[ {{\text{C}}_{2} {\text{mim}}} \right]^{ + }$$ , $$\left[ {{\text{C}}_{4} {\text{mim}}} \right]^{ + }$$ , $$\left[ {{\text{C}}_{6} {\text{mim}}} \right]^{ + }$$ , and stretching vibrations of $${\text{C}}12{-}{\text{H}}3$$ , respectively. By increasing the chain length, the strength of hydrogen bonds decreases as a result of $${\text{C}}12{-}{\text{H}}3$$ bond elongation and less changes are observed in stretching vibrations of $${\text{C}}12{-}{\text{H}}3$$ compared to the free cations. To the best of our knowledge, this research is the first work which reports the far-IR of $$\left[ {{\text{C}}_{4} {\text{mim}}} \right]\left[ {{\text{NTF}}_{2} } \right]$$ , $$\left[ {{\text{C}}_{6} {\text{mim}}} \right]\left[ {{\text{NTF}}_{2} } \right]$$ , and $$\left[ {{\text{C}}_{8} {\text{mim}}} \right]\left[ {{\text{NTF}}_{2} } \right]$$ and the mid-IR of $$\left[ {{\text{C}}_{8} {\text{mim}}} \right]\left[ {{\text{NTF}}_{2} } \right]$$ .