Hydrogen bond in imidazolium based protic and aprotic ionic liquids

Liquid structure of bis-(trifluoromethanesulfonyl)amide TFSA− based protic and aprotic ionic liquids composed of imidazolium [h2Im+], N-methylimidazolium [C1hIm+] and N,N'-dimethylimidazolium [C1mIm+] were investigated by high-energy total scattering (HETS) experiments. The nearest neighboring cation-anon orientation variations by the N-methyl groups substitution to proton were suggested based on the peaks at around 6 and 9 Å in the differential radial distribution functions as the form of r2{GX-ray(r) - 1} for these ionic liquids. It was supposed that the NH · · · O hydrogen bond causes the cation-anion orientation variations. To obtain further insight into the hydrogen bond in the PIL, MD simulations performed and agreed well with the experiments. According to spatial distribution functions (SDF) for the three ionic liquids, the O atom of TFSA− prefers the NH hydrogen of the imidazolium that has the most positive partial atomic charge in the cation, while the F atom locates right above and right below the imidazolium ring plane. In addition, the NH · · · O hydrogen bond has short bond lengths and linear bond angles, while the C2H · ·O interaction is long and bent. The NH · · · O hydrogen bond in the PIL was discussed based on structural aspect accompanied by a thermodynamic viewpoint.