Pressure-Induced Phase Transitions in Ammonium Squarate: A Supramolecular Structure Based on Hydrogen-Bonding and π-Stacking Interactions

Abstract

We report the results of high-pressure Raman and X-ray diffraction measurements performed on ammonium squarate ((NH(4))(2)C(4)O(4), AS), a representative supramolecular architecture based on hydrogen bonding and π-stacking interactions, at various pressures up to 19 GPa. Two phase transitions at ∼2.7 GPa and in the pressure range of 11.1-13.6 GPa were observed. Both Raman and XRD results provide convincing evidence for these two phase transitions. The first phase transition is attributed to the rearrangements of hydrogen-bonding networks, resulting in the symmetry transformation from P2(1)/c to P1. The second one, which is identified as an order-disorder phase transition, arises from significant modifications of squarate rings and random orientations of NH(4)(+) cations. The cooperative effects between hydrogen-bonding and π-stacking interactions, as well as mechanisms for the phase transitions, are discussed by virtue of the local structure of AS.

Compression studies of face-to-face π-stacking interaction in sodium squarate salts: Na2C4O4 and Na2C4O4•3H2O

“…The splitting of ring modes indicates that the squarate ions have been distorted gradually by the external increasing stress from ambient to 11.1 GPa. 22 Meanwhile, the significant changes of all the ring modes at 11.1 GPa are in line with the proposed phase transition. For the ν(CC) mode in this region, it does not exhibit remarkable changes except the band splitting, shown in the plot of 11.1 GPa.…”

“…However, it can be seen from the Raman patterns, the variations in AS is much more obvious than in SS and SST (Raman patterns of AS can be seen in Ref. 22, Figures 2 and 3). That means the distortion degree in AS is the maximum of the three.…”

“…of π -stacking is always overwhelmed or affected by other interactions, it is difficult to detect the unique high-pressure behaviors of π -stacking. 22 Additionally, among the three kinds of stacking interaction (offset, face-to-face, and edge-to-face), face-to-face π -stacking is the rarest one, due to its repulsive nature. 23 These two points lead to the fact that most highpressure studies cannot obtain the unique behaviors of faceto-face π -stacking.…”

Pressure-Induced Magnetic Crossover Driven by Hydrogen Bonding in CuF2(H2O)2(3-chloropyridine)

“…Molecule-based materials are particularly revealing in this regard due to their low energy scales and sensitivity to various external stimuli like temperature, pressure, and magnetic field3456789101112. Another characteristic of molecule-based materials is their tendency to develop hydrogen bonding networks13141516171819202122232425262728. Pressure and strain are especially attractive tuning parameters in this case because they act directly on bond lengths and angles as well as the hydrogen bonding pattern.…”

“…While magnetic exchange interactions are traditionally established through direct and superexchange mechanisms between metal centers or metal centers and various ligands29, exchange can also occur through intermolecular hydrogen bonding141516171819202122232425262728. Physical examples include coordination polymers and low-dimensional systems like Cu(pyz)(NO 3 ) 2 , Cu(pyz)F 2 (H 2 O) 2 , and CuX 2 (pyrazine-N,N′-dioxide) (H 2 O) 2 (X = Cl, Br), all of which display transitions that involve hydrogen bonding networks.…”