Rotational correlation times, diffusion coefficients and quadrupolar peaks of the protic ionic liquid ethylammonium nitrate by means of 1H fast field cycling NMR relaxometry

The protic ionic liquid (PIL) ethylammonium nitrate (EAN) is characterized by hydrogen bonding between the N-H bonds of the cations and the oxygen atoms of the anions. The three-dimensional H-bond network of EAN very much resembles that of water. Although this first ionic liquid is known for more than 100 years, there is still lack of information about the rotational and translational dynamics, and how both are related to each other. For that purpose, we measured the H-1 nuclear magnetic relaxation dispersion curves (NMRD) of EAN by means of Fast Field Cycling (FFC) NMR relaxometry. We were able to analyze the measured H-1 spinlattice relaxation rates R-1 covering a large temperature range between 248 and 333 K simultaneously. Applying the well-known Bloembergen-Purcell-Pound (BPP) approach, we decomposed the total relaxation rates into intramolecular and intermolecular contributions, describing the rotational and translational dynamics of the ethylammonium cation. Here, we report rotational correlation times, tau(R) and translational diffusion coefficients, D-T, both derived from the total H-1 spin-lattice relaxation rates as a function of temperature and frequency. Self-diffusion coefficients, D-T were also determined from the total relaxation rates in the low frequency range, wherein only the intermolecular relaxation contribution is frequency dependent. Then, D-T results from the slope of a linear fit of the spin-lattice relaxation rate R-1 as a function of the square root of frequency. nu. The diffusion coefficients obtained from both methods are in good agreement with self-diffusion coefficients measured by pulsed-field-gradient (PFG) NMR. The rotational correlation times, tau(R), derived from the intramolecular H-1 relaxation contribution were compared to correlation times obtained from high-field NMR deuteron relaxation experiments, dielectric spectroscopy (DS) and femto-second-infrared (fs-IR) spectroscopy. For EAN, we also observed a local enhancement of the H-1 spin-lattice relaxation rates R-1 at high frequencies. This so-called quadrupole relaxation enhancement (QRE) results from the interference of the Zeeman transition energy of the H-1 spins and the energy difference of two levels of the quadrupole N-14 nuclei. QRE is usually known for solids, but rarely for the liquid state as observed here for EAN. Overall, we show that FFC relaxometry provides access to rotational correlation times, translational diffusion coefficients and quadrupole relaxation enhancement at the same time. (C) 2020 Elsevier B.V. All rights reserved.