Hydrogen bonds (HBs) play a key role in the structure and function of biological molecules, the strength of materials, and molecular binding. In Ionic liquids (ILs) solely consisting of cations and anions, different types of H-bonding are important key factors for controlling their properties. In particular, hydroxyl-functionalised cations allow anchoring molecular components with HB donating and accepting capabilities to the cations. This is a good example of how functionalisation can be used as a novel way of adding specific chemical interactions, thus offering the possibility to fine-tune physical and chemical properties. Moreover, functionalisation offers the possibility that cations themselves may form hydrogen-bonded contacts. The resulting cation clusters are stabilised via localised HB networks. This concept of attraction between like-charged ions could be proven by a variety of spectroscopic techniques and is especially pronounced in 1-(4-hydroxybutyl)-pyridinium bis(trifluoromethanesulfonyl)imide, [HOC₄Py][NTf₂].
We can characterize the nature of H-bonds in ILs with a combination of neutron diffraction experiments and molecular dynamics simulations. Interestingly, the HBs between cations seem to be stronger than their cation-anion counterparts, despite Coulomb interactions that are repulsive in the first and attractive in the latter case. The methodology further allows an exploration of the influence of an OH-catcher such as DMSO as an OH-catcher on the delicate balance of HBs. DMSO acts as an OH-acceptor through its strongly proton-accepting S=O group. We observe three types of HB situations. Their strengths are indicated by characteristic bond lengths r_HO and r_OO and HB-lifetimes. The conceptual knowledge allows us to anchor DMSO within the IL and to control nano-scale structures.

Johanna Busch

Universitat Rostock Mathematisch-Naturwissenschaftliche Fakultat

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