Phonons mediate detection of intermolecular forces in ionic liquids

Phonons mediate detection of intermolecular forces in ionic liquids

Issued: Wed, 12 Jul 2017 14:33:00 BST

Ionic liquids are molten salts that have relatively low melting temperatures because of the use of large and “floppy” ions. They have become popular for applications in synthesis, catalysis, electrochemistry, batteries, and—in the case of protic ionic liquids—hydrogen-based fuel cells. The properties of ionic liquids are strongly dependent on intermolecular forces, in particular, coulombic and hydrogen-bonding interactions. The role of hydrogen bonding has remained bit of a mystery because the spectral bands in Raman or IR spectra that could tell you something about them are in the gigahertz and terahertz range, where there are many other overlapping bands from a range of molecular motions. Now the team led by Prof. Klass Wynne have used a trick by using highly symmetric ions in which many of the spectral contributions are “switched off”. By gradually and systematically reducing the symmetry again, they could glean more information about those hydrogen bonds in protic ionic liquids.

Pof Klaas adds: "We found for the first time that ionic liquids have optical phonon modes, that is, terahertz-frequency sound-like waves that are delocalised throughout the liquid. These modes split into distinct longitudinal and transverse optical (LO and TO) phonons in the presence of hydrogen bonding. This shows that protic ionic liquids share many properties with liquid water. We could also show that the optical-phonon bands will be important in the spectra of all ionic liquids, requiring a reinterpretation of all ionic liquid spectra (including, sadly, our own…). On the upside, this provides a future handle to quantify the hydrogen-bonding and coulombic interactions in ionic liquids."

The work was published in the Journal of the American Chemical Society: J. Reichenbach, S.A. Ruddell, M. González-Jiménez, J. Lemes, D.A. Turton, D.J. France, and K. Wynne, “Phonon-like hydrogen-bond modes in protic ionic liquids”, JACS 139, 7160–7163 (2017). ‌

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