Hydrogen bonds structure and phase transitions
in the N-octyl-N,N-diethylammonium tetrafluoroborate [N₂₂₈][BF₄]) protic ionic liquid electrolyte probed by solid-state ²H NMR

Protic ionic liquids (PILs) are promising candidates for electrochemical applications due to their wide liquid-state range, tunable physicochemical properties, and the possibility of controlling hydrogen-bond networks via molecular design. In this work, we report the synthesis and characterization of N,N-diethyl-octan-1-ammonium tetrafluoroborate selectively deuterated at the N—H site, [N₂₂₈D][BF₄]. The phase behavior and local hydrogen-bond structure were investigated by combining solid-state ²H nuclear magnetic resonance (NMR) spectroscopy and differential scanning calorimetry (DSC). DSC revealed the absence of a glass transition in the –120 to +150 °C interval, instead identifying multiple solid-solid transitions and a melting event via intermediate mesophases. Temperature-dependent ²H NMR spectra demonstrated the coexistence of two distinct hydrogen bonding environments in the solid phases (DQCC = 151–177 kHz and 129–140 kHz) at low and intermediate temperatures, with the transition to an isotropically mobile phase above 283 K. The ²H NMR evidenced the presence of the plastic phase above 283 K and enabled quantitative determination of transition thermodynamics (DH⁰ = 59±8 kJ/mol; DS⁰ = 162±25 kJ/(mol×K)) to the state with mobile cations. The observed intermediate DQCC values reflect hydrogen-bond strengths in [BF₄]^– PILs and evidences multiple hydrogen-bonding geometries despite anion symmetry. These findings contribute to the understanding of structure–dynamics–phase behavior relationships in asymmetric cation PILs and may inform the design of optimized electrolytes for energy-related applications.

This is an excerpt of the article

Hydrogen bonds structure and phase transitions in the
N-octyl-N,N-diethylammonium tetrafluoroborate ([N₂₂₈][BF₄])
protic ionic liquid electrolyte probed by solid state ²H NMR.

Full text of the paper is published in Journal of Structural Chemistry.
DOI: 10.1134/S0022476626010117