Phonon-like Hydrogen-Bond Modes in Protic Ionic Liquids

Judith Reichenbach,Mario González-Jiménez,David J France,Julio Lemes,Stuart A Ruddell,David A Turton,Klaas Wynne

doi:10.1021/jacs.7b03036

Judith Reichenbach, Mario González-Jiménez + Show 5 more

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https://doi.org/10.1021/jacs.7b03036

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Abstract

Gigahertz- to terahertz-frequency infrared and Raman spectra contain a wealth of information concerning the structure, intermolecular forces, and dynamics of ionic liquids. However, these spectra generally have a large number of contributions ranging from slow diffusional modes to underdamped librations and intramolecular vibrational modes. This makes it difficult to isolate effects such as the role of Coulombic and hydrogen-bonding interactions. We have applied far-infrared and ultrafast optical Kerr effect spectroscopies on carefully selected ions with a greater or lesser degree of symmetry in order to isolate spectral signals of interest. This has allowed us to demonstrate the presence of longitudinal and transverse optical phonon modes and a great similarity of alkylammonium-based protic ionic liquids to liquid water. The data show that such phonon modes will be present in all ionic liquids, requiring a reinterpretation of their spectra.

Highlights

Gigahertz- to terahertz-frequency infrared and Raman spectra contain a wealth of information concerning the structure, intermolecular forces, and dynamics of ionic liquids
Ionic liquids are organic salts with melting points closer to those of traditional solvents than purely inorganic salts. They retain the useful physical properties associated with molten salts, such as low vapor pressure and a wide electrochemical window. This combination makes them candidates for applications in synthesis, catalysis, electrochemistry, and battery development.[1−4] Protic ionic liquids, which are capable of forming hydrogen bonds, are currently under intense investigation as electrolytes in batteries and hydrogen-based fuel cells.[1,3]
Spectroscopic studies have been carried out on ionic liquids in this frequency domain using far-infrared and terahertz time-domain spectroscopy (THz-TDS)[5−8] as well as Raman and optical Kerr effect (OKE) spectroscopy.[9−12] there are many other motions besides those associated with hydrogen bonding that give rise to spectral bands in this range, including librations, low-frequency intramolecular vibrations, and orientational relaxation

Summary

Introduction

Gigahertz- to terahertz-frequency infrared and Raman spectra contain a wealth of information concerning the structure, intermolecular forces, and dynamics of ionic liquids. Spectroscopic studies have been carried out on ionic liquids in this frequency domain using far-infrared and terahertz time-domain spectroscopy (THz-TDS)[5−8] as well as Raman and optical Kerr effect (OKE) spectroscopy.[9−12] there are many other motions besides those associated with hydrogen bonding that give rise to spectral bands in this range, including librations, low-frequency intramolecular vibrations, and orientational relaxation.

Results

Conclusion