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

Abstract

Diethylammonium nitrate, [N_{0 0 2 2}][NO₃], and its perdeuterated analogue, [N _{D 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.

Figure 1

(a–c) The crystal packing of the diethylammonium nitrate crystal [N_0 0 2 2][NO₃] (orthorhombic, Pmmn), according to low-temperature (120 K) SXRD measurements, shown in alternative 2 × 2 × 2 supercell projections (along the a, b, and c crystallographic axes, respectively), with the (putative) hydrogen bonds marked as dashed blue lines (with O···H distances below 2 Å). (d) Projection of the unit-cell along the b-axis according to the low-temperature SXRD measurements. The heavy atoms are shown as spheres and are superimposed with the PXRD refinements (298 K) to illustrate cell expansion with temperature. The cell constants at both temperatures are shown in Å in the inset (room-temperature values are given in parentheses).

Figure 2

Powder diffraction patterns of [N_0 0 2 2][NO₃]. Low-resolution patterns were recorded using a SXRD instrument. The high-resolution diffractogram was recorded at room temperature using a standard setup. The bottom panel shows the simulated diffractogram from the single-crystal structure solved at 120 K. The asterisks in the low-temperature diffraction data suggest the presence of a fraction of molecules with the gauche conformation. This finding has been confirmed by model simulations assuming a 25% contribution from the alternative conformers in the lattice (blue patterns in the bottom panel; see further discussion in the main text).

Figure 3

Experimental and theoretical (harmonic lattice dynamics, PBE/1050 eV/hard-NCPPs) IR, INS, and Raman spectra of hydrogenous (↑) [N_0 0 2 2][NO₃] and perdeuterated (↓) [N_0 0 D D][NO₃] diethylammonium nitrate crystals in the high- (3250–1700 cm^–1) and intermediate-energy (1700–600 cm^–1 on the log scale) range. The characteristic N–H···O vibrations are labeled as (a–d) and highlighted with vertical shaded bars, color-coded according to the spectroscopic technique most sensitive to a given spectral feature (red for IR; grey for INS; and blue for Raman).

Figure 4

Hydrogen-projected VDoSs of hydrogenous [N_0 0 2 2][NO₃] obtained from ab initio MD simulations (PBE/1050 eV/hard-NCPP) in the microcanonical ensemble at selected temperatures. Left: the top panel shows the experimental IR and INS spectra. The bottom panel presents total contributions from hydrogens to the VDoS. The characteristic N–H···O vibrations are labeled as (a–d) and highlighted with vertical shaded bars, as in previous figures. Right: the top panel shows the experimental IR spectrum in the high-energy range. The bottom panel presents hydrogen-projected partial VDoSs for the [NH₂] moiety.

Figure 5

(Spectra on the left) Experimental and theoretical (harmonic lattice dynamics, PBE/1050 eV/hard-NCPPs) INS spectra of hydrogenous (↑) [N_0 0 2 2][NO₃] and perdeuterated (↓) [N_0 0 D D][NO₃] diethylammonium nitrate crystals in the intermediate- (1700–500 cm^–1; the left panel) and low-energy regime (500–24 cm^–1; the right panel), respectively. The theoretical spectra come from two different models, a perfect Pmmn crystal (all-trans, primitive-cell calculations) and partially disordered supercell model accounting for conformational changes in the powder specimen (25% gauche). The shaded area in cyan highlights the spectral range particularly sensible to conformational changes. The gray shaded area highlights the external modes. (Structural models on the right) Crystal voids are shown in brown over the 2 × 2 × 2 supercell of the crystallographic Pmmn model (all-trans), and compared to a model accounting for a 25% contribution from CH₂CH₃ chains in gauche conformation (P1 symmetry).