Compression studies of face-to-face pi-stacking interaction in sodium squarate salts: Na2C4O4 and Na2C4O4 center dot 3H(2)O

High-pressure Raman scattering and synchrotron X-ray diffraction measurements of sodium squarate (Na2C4O4, SS) are performed in a diamond anvil cell. SS possesses a rare, but typical structure, which can show the effect of face-to-face pi-stacking without interference of other interactions. At similar to 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 pi-stacking that dominates the distortion. For comparison, high-pressure Raman spectra of sodium squarate trihydrate (Na2C4O4 center dot 3H(2)O, SST) are also investigated. The structure of SST is determined by both face-to-face pi-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 similar to 10.3 GPa. Our results can be well supported by the previous high-pressure studies of ammonium squarate ((NH4)(2)C4O4, 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. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4767057]