Ionic Mobility in the N-Octyl-N,N-diethylammonium Tetra-fluoroborate ([N228][BF4]) Protic Ionic Liquid in Solid and Liquid States Probed by Solid-State 2H NMR

Abstract

The cation structure plays a crucial role in determining the properties and consequently the potential applications of ionic liquids. Understanding how cation structure can govern ionic mobility is essential for optimizing their performance in a wide range of fields. In this work, we study the ionic mobility of a tri-alkyl-ammonium-based protic ionic liquid (PIL) featuring an asymmetric cation paired with tetra-fluoroborate anion by means of ²H NMR spectroscopy. By applying solid-state ²H NMR spectroscopy on the perdeuterated N–H(N–D) group of the ammonium cation over a wide temperature range from 123 to 463 K, we have monitored the cations’ dynamics in both solid and liquid states. The molecular dynamics of the PIL were quantified through the analysis of spin–lattice (T₁) and spin–spin (T₂) relaxation times. ²H NMR spectra line shape analysis was additionally used to quantify the un-averaged deuterons quadrupolar coupling constant (DQCC = 177 kHz at 123 K) of the N–D deuteron in this PIL. The spin relaxation analysis yielded the activation barriers and pre-exponential factors for isotropic and anisotropic motion, providing detailed insights into the dynamical behavior of [N₂₂₈][BF₄]. Our findings demonstrate that cation asymmetry benefits ionic mobility, suggesting that [N₂₂₈][BF₄] is a promising candidate for electrolyte applications.