The solid state, surface and morphological properties of p-aminobenzoic acid in terms of the strength and directionality of its intermolecular synthons
Empirical force-field calculations utilising the atom–atom method were used to examine the strength, directionality and chemical state of the intermolecular interactions (synthons) present in the polymorphic forms (α and β) of p-aminobenzoic acid (pABA). This is set within the context of predicting the morphology of both forms in terms of the unsatisfied synthons at each growth surface. The α lattice energy was calculated to be −24.54 kcal mol−1 with the dominant intermolecular interactions found to consist of OH⋯O carboxylic acid H-bonding dimers and head to head π–π stacking interactions. The β lattice energy was calculated to be −22.73 kcal mol−1 and the dominant interactions found to consist of a 4-membered H-bonding ring made up of two identical NH⋯O and OH⋯N interactions, plus strong head to tail π–π stacking interactions. The NH2 group was calculated to contribute more to the β lattice energy than to the α, as it acts as a H-bonding donor and acceptor in the β structure, whilst acting solely as a donor in α. Conversely, the COOH group was found to contribute more strongly to the α lattice energy due to the formation of the OH⋯O H-bonds and also NH⋯O H-bonds, while the COOH group in the β structure forms only weaker O⋯HN and OH⋯N interactions. Morphological prediction of the β form gave greater resemblance to the experimental morphology compared to α. Surface chemistry analysis revealed that the strength, character and directionality of the synthons present varies in terms of their anisotropy between these two polymorphs. The strength and character of the unsaturated synthons exposed at the major surfaces of the α crystal were found to significantly vary, which results in a needle-like morphology. In contrast, the strength and character of the synthons exposed at the major surfaces of the β morphology were found to be much more similar, which results in the more equant morphology. Overall, this paper presents a synthonic, analytical approach which holistically links the molecular properties with the bulk and surface synthons, and through this rationalises their contributions to the growth and morphology of this organic crystalline system.