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

Abstract

High-pressure Raman scattering and synchrotron X-ray diffraction measurements of sodium squarate (Na(2)C(4)O(4), SS) are performed in a diamond anvil cell. SS possesses a rare, but typical structure, which can show the effect of face-to-face π-stacking without interference of other interactions. At ~11 GPa, it undergoes a phase transition, identified as a symmetry transformation from P2(1)/c to P2(1). From high-pressure Raman patterns and the calculated model of SS, it can be proved that the phase transition results from the distorted squarate rings. We infer it is the enhancement of π-stacking that dominates the distortion. For comparison, high-pressure Raman spectra of sodium squarate trihydrate (Na(2)C(4)O(4)●3H(2)O, SST) are also investigated. The structure of SST is determined by both face-to-face π-stacking and hydrogen bonding. SST can be regarded as a deformation of SS. A phase transition, with the similar mechanism as SS, is observed at ~10.3 GPa. Our results can be well supported by the previous high-pressure studies of ammonium squarate ((NH(4))(2)C(4)O(4), AS), and vice versa. High-pressure behaviors of the noncovalent interactions in SS, SST, and AS are compared to show the impacts of hydrogen bonding and the role of electrostatic interaction in releasing process.

Hydrogen-Bonded Cyclic Dimers at Large Compression: The Case of 1H-pyrrolo[3,2-h]quinoline and 2-(2′-pyridyl)pyrrole

“…This leads to an increase in the stacking interaction between pairs of molecules that are not hydrogen-bonded. A similar increase in stacking interactions was observed in a number of systems [39][40][41]. Most notably, in the case of pyrrole, compression leads to formation of a crystal structure consisting of dimers exhibiting short C•••C contacts [41].…”

Structural Tuning and Piezoluminescence Phenomenon in Trithiocyanuric Acid

“…At 3.2 GPa, new bending modes of δ­(CS) and δ­(NH) appeared, indicating the beginning of the conformational distortion of the planar TTCA molecules. During further compression, the molecules became further distorted, as evidenced by the emergence of some additional δ­(CS) and δ­(NH) modes at 6.7 and 7.3 GPa, respectively . The Raman peaks marked by the asterisks in Figure a shifted from the lower-frequency region.…”

“…During further compression, the molecules became further distorted, as evidenced by the emergence of some additional δ(CS) and δ(NH) modes at 6.7 and 7.3 GPa, respectively. 23 The Raman peaks marked by the asterisks in Figure 3a shifted from the lower-frequency region. In addition, the out-of-plane ring vibration modes, δ(ring), exhibited continuous red shifts with increasing pressure (Figure 3b), which is ascribed to the enhanced attractive interactions between the molecular layers during their gradual approach.…”

“…21,22 Layered structures normally have an interlayer direction with weak noncovalent interactions, so that an external force can efficiently decrease the intralayer distances and introduce relative slippage between layers. 23,24 The decreased distance and slippage between molecular layers lead directly to the destruction of types of interactions, as well as the construction of the new crystal and electronic structures.…”

“…Among molecular crystals, layered architectures always exhibit intriguing structural patterns and high-pressure properties. , Layered structures normally have an interlayer direction with weak noncovalent interactions, so that an external force can efficiently decrease the intralayer distances and introduce relative slippage between layers. , The decreased distance and slippage between molecular layers lead directly to the destruction of types of interactions, as well as the construction of the new crystal and electronic structures. Based on these considerations, we chose one canonical example of a perfectly layered structure, trithiocyanuric acid (C 3 H 3 N 3 S 3 , TTCA), as the model to explore the effects of pressure on its interactions, structure, and properties.…”