Is the ionic liquid ethylammonium nitrate like water?

Abstract

The protic ionic liquid ethylammonium nitrate [EtNH3+][NO3–] (EAN) can in some ways be considered as an archetypal IL. Certainly, since its first investigation 100 years ago by Walden, it is one of the best known. The study of the dynamics of EAN, with its relatively simple structure, provides an opportunity to better understand the behaviour of more complex ILs. However, there is another reason why EAN warrants special attention: this is because it has been proposed as an analogue of that truly mysterious but very important substance dihydrogen monoxide (aka water). The basis for this claim is an apparent similarity between various H-bonding modes that are observed in the far-infrared (THz) region of both substances. However, such spectra are difficult to measure and even more difficult to interpret without detailed knowledge of other possible modes that might exist in this spectral region. This talk will describe the dynamics of EAN and some closely-related compounds over the extraordinarily wide frequency range from 200 MHz to 10 THz (~350 cm–1) obtained by using a combination of dielectric relaxation and optical Kerr-effect spectroscopies. Detailed analysis of these spectra shows that the observed intensities are actually a complex combination of overlapping and probably coupled modes, including H-bond vibrations, cation and anion librations and possibly other cooperative effects; the relationships between EAN and water are not clear-cut