Collapse of the Helical Hydrogen Bond Structure in Imidazolium Hydrogen o-Phthalate Under High Pressure: A Vibrational Spectroscopy Study.

Imidazolium hydrogen o-phthalate (OrtImi) is an organic ionic crystal with a helical hydrogen-bonded network. We report on the evolution of the Raman, terahertz, and infrared modes of OrtImi over a wide range of temperatures and hydrostatic pressures. We interpret the results using density functional theory and molecular dynamics within machine-learning interatomic potential framework calculations. The vibrational spectra uncovered a phase transition occurring above 3 GPa applied hydrostatic pressure. This transition is associated with a collapse of the helical network of N+-H···O- hydrogen bonds transforming into a ring motif within the ab plane. Proton ordering in hydrogen bonds emerges upon cooling below 150 K, creating favorable conditions for proton-phonon coupling.