The Influence of the Intermolecular Synthons on the Molecular Aggregation, Polymorphism, Crystal Growth and Morphology of p-Aminobenzoic Acid Crystals from Solution

Intermolecular pairwise interactions (synthons) have previously been used to predict physical properties of molecular crystals, such as polymorphism, morphology and mechanical properties. The overall goal of this research was to correlate the synthon strength and nature found in the crystal structures of the α- and β-forms of 4-aminobenzoic acid to experimentally observed data, assessing their applicability for predicting crystallisation conditions that would produce crystals of desirable physical properties.
The strength and nature of the synthons found in the α-structure were found to significantly vary, while the strength and nature of the synthons in the β-structure were found to be much more similar. These synthons were also found to influence the conformation of the COOH and NH2 groups in both structures.
Water was calculated to favourably bind to the COOH group than ethanol, along with the OH...O H-bonding dimers being calculated to be less stable in aqueous than ethanolic solutions. This suggests that water may hinder the formation of the α form more so than other solvents.
The attachment energy morphological prediction of α-pABA was less needle-like than often observed, probably due to the growth rate of the capping faces growing by a rough interfacial growth mechanism along the long axis of the needle dominated by π-π stacking interactions, being underestimated. Nitromethane (NMe) was calculated to interact more strongly with the phenyl ring than other solvents and hence addition of NMe to ethanolic solutions shortens the long axis of the needle.
The experimentally measured local surface molecular chemisty was found to vary between measurements of the bulk and surface at the (1 0 -1) surface, which is predicted to have unsaturated H-bonds at the surface. Complementing this, spatially resolved measurements across the (1 0 1) surface also showed changes in structure of a supposedly periodic surface.
This study revealed how predicting the strength and nature of the synthons in the bulk structure and at the surface, along with their interaction with solvent molecules and stability in solution, can influence the morphology and polymorphism of the crystals produced from solution.