Proton transfer aiding phase transitions in oxalic acid dihydrate under pressure

Abstract

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.

Sporopollenin, a Natural Copolymer, is Robust under High Hydrostatic Pressure

“…The rate of decrease of the νOH band can be interpreted as a proxy for the strength of the bond, suggesting that all of the constituent components of the sporopollenin undergo greater changes under pressure than the natural material. Of those studied, only p ‐coumaric acid and sebacic acid show signs of new hydrogen bonding within pure samples.…”

High Pressure Spectroscopic Investigation on Proton Transfer in Squaric Acid and 4,4′-Bipyridine Co-crystal

“…Based on the calculated value, we plot the bond length of N-H and O-H changes as a function of pressure in Figure S5 (Supporting Information), It can be seen that the proton transfer from SQ to BP occurs at about 1.5 GPa suggests that the double proton transfer structure possess the lowest enthalpy under compression, and the first-order phase transition can be associate with the proton transfer 59 . For BP molecule, the two pyridine rings are not coplanar and form a dihedral angle of 34◦ at ambient condition, the calculated dihedral angle decreases with increasing pressure (shown in Figure S6 (Supporting Information)), which is similar to the structural change of BP at high temperature 46 .…”

“…The proton acceptor-BP possesses a non-planar aromatic molecular geometry under ambient conditions. It has been reported that the intensity ratio between ring breathing and C-C stretching mode can correlate with the dihedral angle between two pyridine ring in BP 64–67 . The smaller dihedral angle is, the lower intensity ratio of ring breathing to C-C stretching mode is.…”

Migrating hydrogen in 2,4,6-triaminopyrimidinium(1+) x hydrogen trioxofluorophosphate(−) x monohydrate/2,4,6-triaminopyrimidinium(2+)1–x trioxofluorophosphate(2–)1–x monohydrate (0.0 < x < 0.73) with changing temperature

“…Compound Probe (Casati et al, 2009), as well as by IR spectroscopic methods (Bhatt et al, 2016). As for NÁ Á ÁHÁ Á ÁO hydrogen bonds, there are indications that a similar proton transfer provoked by exerted pressure takes place in the 1:1 adduct of squaric acid and 4,4 0 -bipyridine (Martins et al, 2009).…”