The study of hydrogen bonds near symmetrization limit at high pressures is of importance to understand proton dynamics in complex bio-geological processes. We report here the evidence of hydrogen bond symmetrization in the simplest amino acid-carboxylic acid complex, glycinium oxalate, at moderate pressures of 8 GPa using in-situ infrared and Raman spectroscopic investigations combined with first-principles simulations. The dynamic proton sharing between semioxalate units results in covalent-like infinite oxalate chains. At pressures above 12 GPa, the glycine units systematically reorient with pressure to form hydrogen-bonded supramolecular assemblies held together by these chains.

Dimethyl ammonium (DMA) metal formate, an important member of the dense metal organic framework (MOF) family, is known to exhibit a low temperature ferroelectric transition, caused by the ordering of the hydrogen bonds. In this study, we probed the effect of pressure on the disordered hydrogen bond and the HCOO linkers of DMA manganese formate, with the help of XRD, IR and Raman spectroscopic studies up to ∼20 GPa. We observed that though a phase transition was initiated at ∼3.4 GPa, it was complete only by 6 GPa, indicating its first order nature. Beyond 7 GPa, this compound becomes highly disordered and shows an almost amorphous character, indicating a total collapse of the formate network. The reversibility of the initial structure of DMAMnF on the release of pressure from 20 GPa (i.e. from a highly disordered phase) shows the remarkable resilience of the formate cage. At the first crystal to crystal transition at 3.4 GPa, the distortion of the formate cage causes the ordering of the dynamically disordered hydrogen bond, resulting in a rearrangement of the DMA cation. Lifting of the mutual exclusivity of the Raman and IR modes (C-H out of plane and O-C-O bending modes) of HCOO linkers, at this transition, indicates that the high pressure phase may be non-centro-symmetric.

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Oxalic acid dihydrate, an important molecular solid in crystal chemistry, ecology and physiology, has been studied for nearly 100 years now. The most debated issues regarding its proton dynamics have arisen due to an unusually short hydrogen bond between the acid and water molecules. Using combined in situ spectroscopic studies and first-principles simulations at high pressures, we show that the structural modification associated with this hydrogen bond is much more significant than ever assumed. Initially, under pressure, proton migration takes place along this strong hydrogen bond at a very low pressure of 2 GPa. This results in the protonation of water with systematic formation of dianionic oxalate and hydronium ion motifs, thus reversing the hydrogen bond hierarchy in the high pressure phase II. The resulting hydrogen bond between a hydronium ion and a carboxylic group shows remarkable strengthening under pressure, even in the pure ionic phase III. The loss of cooperativity of hydrogen bonds leads to another phase transition at ∼ 9 GPa through reorientation of other hydrogen bonds. The high pressure phase IV is stabilized by a strong hydrogen bond between the dominant CO2 and H2O groups of oxalate and hydronium ions, respectively. These findings suggest that oxalate systems may provide useful insights into proton transfer reactions and assembly of simple molecules under extreme conditions.