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Hydrogen Bonds of the Imidazolium-ring 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 ([Cnmim][TFSA], n = 2-12 represents the alkyl chain length), with the O atom of dimethyl sulfoxide (DMSO) have been elucidated using 1H, 13C, and 15N NMR and soft X-ray absorption and emission spectroscopy (XAS and XES). Density functional theory (DFT) calculations have been made on isolated DMSO molecule and two cluster models of [Cnmim]+-DMSO by hydrogen bonding to interpret the XES spectra for [Cnmim][TFSA]-DMSO solutions. The 1H and 13C NMR chemical shifts of the imidazolium ring showed that the deshielding of the ring H and C atoms is moderate against the increase in DMSO mole fraction xDMSO to ∼0.8, while that becomes more significant with further increasing xDMSO. This finding suggests that the hydrogen bonds of the three ring H atoms with the DMSO O atoms are saturated in the solutions with increasing xDMSO to ∼0.8. The 1H and 13C chemical shifts of the alkyl chain revealed that the electron densities of the chain H and C atoms are gradually decreased with increasing xDMSO, 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 with the alkyl chain is shielded with the increase in the xDMSO range from 0 to 0.8, and then deshielded with further increasing xDMSO. In contrast, the imidazolium ring N3 atom is simply deshielded with increasing xDMSO. Thus, the electron densities of the alkyl chain may be condensed at the C7 and N1 atoms of [Cnmim]+ by the hydrogen bonding of the ring H atoms with DMSO. The hydrogen bonding of DMSO with the ring brings the low-energy shifts of the XES peaks of the O K-edge of DMSO. Small-angle neutron scattering experiments showed that [Cnmim][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.

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Publication details

The article was received on 10 Feb 2018, accepted on 11 Apr 2018 and first published on 12 Apr 2018

Article type: Paper
DOI: 10.1039/C8CP00963E
Citation: Phys. Chem. Chem. Phys., 2018, Accepted Manuscript
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    Hydrogen Bonds of the Imidazolium-ring of Ionic Liquids with DMSO Studied by NMR, Soft X-ray Spectroscopy, and SANS

    T. Takamuku, T. Tokuda, T. Uchida, K. Sonoda, B. A. Marekha, A. Idrissi, O. Takahashi, Y. Horikawa, J. Matsumura, T. Tokushima, H. Sakurai, M. Kawano, K. Sadakane and H. Iwase, Phys. Chem. Chem. Phys., 2018, Accepted Manuscript , DOI: 10.1039/C8CP00963E

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