Amphidynamic Molecular Crystal with Temperature-Controlled Helical Hydrogen-Bonded Network: Proton Dynamics and Order–Disorder Phase Transition

Hydrated imidazolium hemimelitate with helical hydrogen bonding network is the first amphidynamic organic crystal observed in a group of imidazolium and carboxylic acid compounds. The sublattice of acid ions forms a static network, while the dynamic part comprises imidazole ions and water molecules. A transition from positional to orientational disorder of water molecules is observed as the temperature closes to room temperature and the spatial arrangement of cations leads to an order-disorder phase transition at a temperature of 150 K, which we analyzed in a wide spectral range using THz, FIR, MIR, and Raman spectroscopies. Furthermore, DFT calculations were employed to understand the molecular dynamics and the phase transition mechanism of the studied compound. The temperature-dependent spectra also revealed proton-phonon coupling to occur below 100 K. Our findings provide valuable information, such as temperature behavior of hydrogen bonds, anharmonicity, and coupling effects for the targeted design of amphidynamic materials.