The modeling the structure of croconic and squaric acids under pressure

Abstract

Quantum mechanical simulations of the structure of croconic and squaric acids under pressure up to 25 GPa predict the formation of a new high-pressure crystalline phases. Density functional theory (DFT) plane-wave calculations were performed to investigate the effects of isotropic compression on the structural transformations of croconic and squaric acids and to elucidate the details of the phase transitions The advantage of the method is that it allows to analyze a transition path and to monitor changes of bond lengths upon compression and decompression. The onset of pressure-induced polymerization of croconic acid was observed near 12 GPa, with the formation of strong hydrogen (O-H) bonds with a length of approximately 1.25 Å. The bond corresponds to an intermediate type of bonding between weak hydrogen (bond length ∼1.57 Å) and hydroxyl covalent (bond length ∼1.1 Å) bonds. Complete polymerization was noted at pressures near 25 GPa. Further compression up to 55 GPa did not significantly change the O-H bond length (∼1.2 Å), but was found to result in the O-H-O bond angle approaching ∼180 degrees. The simulations revealed a hysteresis upon decompression in the pressure-volume curve as well as the O-H bond lengths indicated possibility of stabilization of high pressure phases of the acids by pressure. Calculated Raman spectra of the acids at the high and ambient pressure are compared with published experimental vibrational spectra. The predicted structures are compared with that obtained using evolutionary method based on DFT plane-wave calculations.