p-Aminobenzoic acid polymorphs under high pressures

Abstract

The effect of high pressure on two forms (α, β) of p-aminobenzoic acids (PABA) is studied in a diamond anvil cell using in situ Raman spectroscopy.

Discovery and recovery of delta p-aminobenzoic acid

“…The samples in ethanol and water:ethanol show no conversion to δ-pABA on compression (Figure 2b&c). Our original hypothesis that enhanced solubility in the pressure medium facilitates the αto δ-pABA transition (in part informed by the absence of transformation when using low-solubility silicone oil 36 ) cannot be reconciled with our observations here. We believe that the viscosity of the solution may be causing difficulties in molecular mobility hence the nucleation of the high-pressure phase.…”

“…Previous work on the α-and β-forms at high pressure using Raman spectroscopy indicated that each of the phases were stable on compression to 13 GPa. 36 The stability of the α-and β-forms was attributed to the favourable dimer and tetramer hydrogen bonding configurations observed in the two phases. Both of these studies were performed in an inert silicone oil medium which may have helped to supress the conversion into the δ-form from either of these initial phases due to the significant molecular rearrangement required.…”

“…Yan et al investigated the pressure dependence of α-and β-pABA (both P21/n) observing that both phases are stable to 13 GPa using an inert pressure-transmitting medium, silicone oil. 36 Through ab initio approaches they calculated the compression of the crystal structure assuming there was no change in phase and attributed the stability of the polymorphs to the hydrogen bonded dimers and tetramers in α-pABA and β-pABA respectively. Following this work, we wanted to explore how pABA would react to compression using different pressure transmitting media in which the solubility of pABA varied from low (water) to high (ethanol).…”

Pressure-induced phase transition in N–H⋯O hydrogen-bonded crystalline malonamide

“…Hydrogen-bonded molecular crystals under high pressure are the subject of extensive research and are ubiquitous phenomena. [16][17][18][19][20][21][22][23] Amides are of particular research interest because they are widespread in nature and have many technological applications. 24,25 Amides have the smallest peptide bonds and are model compounds for the investigation of hydrogen bonds in biologically active complex peptides.…”

High Pressure Structural Investigation of Benzoic Acid: Raman Spectroscopy and X-ray Diffraction

“…A number of investigations on the structural behaviors of hydrogen-bonded molecular crystals under high pressure have been reported. [11][12][13][14][15][16][17][18][19][20] For example, urea experienced four structural changes at 0.48, 0.6, 2.8, and 7.2 GPa, respectively. [17][18] During the phase transitions, large variations were observed in hydrogen bonds, including breakage, formation, and distortion.…”