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

Abstract

The hydrogen bonds of the imidazolium-ring H atoms of ionic liquids (ILs), 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amides ([C_nmim][TFSA], n = 2 to 12 where n represents the alkyl chain length), with the O atom of dimethyl sulfoxide (DMSO) have been elucidated using ¹H, ¹³C, and ¹⁵N 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_nmim]⁺–DMSO by hydrogen bonding to interpret the XES spectra for the [C_nmim][TFSA]–DMSO solutions. The ¹H and ¹³C 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 ¹H and ¹³C 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 N¹-bound carbon atom C⁷ of the chain. The ¹⁵N NMR chemical shifts showed that the imidazolium-ring N¹ 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 N³ atom is simply deshielded with increasing x_DMSO. Thus, the electron densities of the alkyl chain may be condensed at the C⁷ and N¹ atoms of [C_nmim]⁺ 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_nmim][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.