Hydrogen bonds of the imidazolium rings of ionic liquids with DMSO studied by NMR, soft X-ray spectroscopy, and SANS

The hydrogen bonds of the imidazolium-ring H atoms of ionic liquids (ILs), 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amides ([C n mim][TFSA], n = 2 to 12 where n represents the alkyl chain length), with the O atom of dimethyl sulfoxide (DMSO) have been elucidated using 1 H, 13 C, and 15 N NMR spectroscopy and soft X-ray absorption and emission spectroscopy (XAS and XES). Density functional theory (DFT) calculations have been performed on an isolated DMSO molecule and two cluster models of [C n mim] + -DMSO by hydrogen bonding to interpret the XES spectra for the [C n mim][TFSA]-DMSO solutions. The 1 H and 13 C NMR chemical shifts of the imidazolium ring showed that deshielding of the ring H and C atoms is moderate as the DMSO mole fraction x DMSO increases to ∼0.8; however, it becomes more significant with further increase of x DMSO . This finding suggests that the hydrogen bonds of the three ring H atoms with the DMSO O atoms are saturated in solutions with x DMSO increased to ∼0.8. The 1 H and 13 C chemical shifts of the alkyl chains revealed that the electron densities of the chain H and C atoms gradually decrease with increasing x DMSO , except for the N 1 -bound carbon atom C 7 of the chain. The 15 N NMR chemical shifts showed that the imidazolium-ring N 1 atom which is bound to the alkyl chain is shielded with increasing x DMSO in the range from 0 to 0.8 and is then deshielded with further increase of x DMSO . In contrast, the imidazolium ring N 3 atom is simply deshielded with increasing x DMSO . Thus, the electron densities of the alkyl chain may be condensed at the C 7 and N 1 atoms of [C n mim] + by the hydrogen bonding of the ring H atoms with DMSO. The hydrogen bonding of DMSO with the ring results in low-energy shifts of the XES peaks of the O K-edge of DMSO. Small-angle neutron scattering experiments showed that [C n mim][TFSA] and DMSO are homogeneously mixed with each other on the mesoscopic scale. This results from the strong hydrogen bonds of DMSO with the imidazolium-ring H atoms. The three sites of [C n mim] + are fully hydrogen-bonded with DMSO, leading to homogeneous mixing.