vibrational assignments, conformational analysis, and molecular structures of [cnmim][ntf2] (n = 2, 4, 6, and 8) imidazolium-based ionic liquids: a combined experimental and quantum chemical approach

This work is aimed at providing physical insights about the interactions of cations, anion, and ion pairs of four imidazolium-based ionic liquids of [cnmim][ntf2] 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 [cnmim][ntf2] ion pairs indicate that hydrogen-bonding interactions between oxygen and nitrogen atoms (more negative charge) on [ntf2]− 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 [c2mim][ntf2] were optimized. according to our results, the strongest and weakest hydrogen bonds were existing in [c2mim][ntf2] and [c8mim][ntf2], respectively. a redshift of 290, 262, 258, and 257 cm−1 has been observed for cations involving [c2mim]+, [c4mim]+,[c6mim]+, and stretching vibrations of c12−h3, respectively. by increasing the chain length, the strength of hydrogen bonds decreases as a result of c12−h3 bond elongation and less changes are observed in stretching vibrations of c12−h3 compared to the free cations. to the best of our knowledge, this research is the first work which reports the far-ir of [c4mim][ntf2], [c6mim][ntf2], and [c8mim][ntf2] and the mid-ir of [c8mim][ntf2].