Hydrogen Bonding in Protic Ionic Liquids: Structural Correlations, Vibrational Spectroscopy, and Rotational Dynamics of Liquid Ethylammonium Nitrate

TL;DR

This study combines simulations and experiments to analyze hydrogen bonding in ethylammonium nitrate, revealing a flexible network with broad IR absorption and rapid N-H bond dynamics, distinguishing it from related ionic liquids.

Contribution

It provides a detailed characterization of hydrogen bond flexibility, vibrational spectra, and rotational dynamics in ethylammonium nitrate using combined computational and experimental methods.

Findings

Hydrogen bonds form a flexible 3D network with moderate strength.

IR absorption band broadening due to network flexibility.

Rapid N-H bond reorientation with a correlation time of 16.1 ps.

Abstract

The properties of the hydrogen bonds in ethylammonium nitrate are analyzed by using molecular dynamics simulations and infrared as well as nuclear magnetic resonance experiments. Ethylammonium nitrate features a flexible three-dimensional network of hydrogen bonds with moderate strengths, which makes it distinct from related triethylammonium-based ionic liquids. First, the network's flexibility is manifested in a not very pronounced correlation of the hydrogen bond geometries, which is caused by rapid interchanges of bonding partners. The large flexibility of the network leads to a substantial broadening of the mid-IR absorption band, with the contributions due to N-H stretching motions ranging from 2800 to 3250~\cm. Finally, the different dynamics are also seen in the rotational correlation of the N-H bond vector, where a correlation time as short as 16.1~ps is observed.