Spectroscopic Signatures of Hydrogen-Bonding Motifs in Protonic Ionic Liquid Systems: Insights from Diethylammonium Nitrate in the Solid State

Diethylammonium nitrate,
[N0 0 2 2]­[NO3], and its perdeuterated
analogue, [ND D 2 2] [NO3], were structurally characterized
and studied by infrared, Raman, and inelastic neutron scattering (INS)
spectroscopy. Using these experimental data along with state-of-the-art
computational materials modeling, we report unambiguous spectroscopic
signatures of hydrogen-bonding interactions between the two counterions.
An exhaustive assignment of the spectral features observed with each
technique has been provided, and a number of distinct modes related
to NH···O dynamics have been identified. We put a particular
emphasis on a detailed interpretation of the high-resolution, broadband
INS experiments. In particular, the INS data highlight the importance
of conformational degrees of freedom within the alkyl chains, a ubiquitous
feature of ionic liquid (IL) systems. These findings also enable an
in-depth physicochemical understanding of protonic IL systems, a first
and necessary step to the tailoring of hydrogen-bonding networks in
this important class of materials.