Ionic liquids (ILs) have attracted a phenomenal amount of attention over the past two decades, but the focus has been principally on the development of industrial applications and sustainable processes. Although many theoretical models of IL structures have been developed and published, we are still a long way from predictive certainty, or even good qualitative models for describing the principle interionic interactions.
The presence and nature of hydrogen bonding (H-bonding) in ILs was originally controversial, but now it is generally accepted. This work focuses on protonic ILs, and the search for spectroscopic signatures of H-bonds that have a significant impact on physicochemical properties. The term protonic ILs (PILs) is used to distinguish strongly acidic cations (those which contain readily dissociable protons) from protic ILs, which contain very weakly acidic protons.
As ILs consist only of anions and cations, it is generally assumed that Coulombic interactions dominate the properties of these compounds. However, the determination of H-bonded environments is critical for the understanding of how these interactions influence lattice energies, melting points, and the general behaviour of ILs. It has been estimated that, for organic salts, the H-bonds may contribute up to 25% of the lattice energy.
The H-bonds in a protonic IL will always be stronger than cation−neutral or anion−neutral H-bonds, with neutral−neutral being the weakest. In line with the above, the primary objective of this work was to search for signatures of interionic H-bonds in PILs using state-of-the-art experimental and computational methods. The study was a product of the collaboration between Queen's University of Belfast, UK; Materials Physics Centre & Donostia International Physics Centre in San Sebastian, Spain and ISIS.
Key to their study, published in The Journal of Physical Chemistry C, was the synthesis of the perdeuterated analogue, as the position of the D···O bands is expected to occur at significantly different frequencies than for H···O. The researchers studied both protonated and perdeuterated PILs using X-ray diffraction, calorimetry, and complementary IR, Raman, and high-resolution inelastic neutron spectroscopy.
Neutron scattering was a useful method, due to the interest in low-energy vibrational excitations. The group used (incoherent) inelastic neutron scattering (INS), using the TOSCA instrument at ISIS. Unlike the complementary techniques of IR and Raman spectroscopy, INS exhibits no optical selection rules, thus providing direct access to nuclear motions. Particularly, the researchers took advantage of its exquisite sensitivity to proton dynamics in the region below 1000 cm−1.
This study reports, for the first time, an exhaustive, computationally supported study of PILs using INS spectroscopy. The researchers identified distinct modes associated with cation−anion interactions and clarified the structural peculiarities in the crystal structures induced by the conformational freedom of the constituent molecules.
A joint analysis of the low- and high energy spectral regimes performed by means of optical and neutron spectroscopies allowed them to classify the observed H-bonding motifs as being of moderate strength. An exhaustive analysis of the INS data allowed the group to identify the signatures of hydrogen-bonding well within the terahertz-regime. External modes were found to be equivalent for both ILs, indicating that isotopic effects have no noticeable influence on the crystal structure.
The wealth of experimental data collected in this work could further serve as a high-quality benchmark for much-needed developments of accurate classical force-fields for PILs.
This work is dedicated to the memory of a friend and colleague, Professor Kenneth R. Seddon- a pioneer of ionic-liquid research and initiator of this project.
The full paper can be found at DOI: 10.1021/acs.jpcc.1c05137
