Conformational and structural stability of the single molecule and hydrogen bonded clusters of para aminobenzoic acid in the gas and solution phases

The crystallographic structures of the α- and β- polymorphic forms of para aminobenzoic acid are deconstructed into their constituent hydrogen bonding molecular structural building blocks of monomers, dimers, tetramers and octamers, where they are analysed using ab initio quantum mechanical calculations of their conformation and cluster stability in solution. The molecular conformation found in the β-form is less stable than the same found in the α-form for both the gas and solution phases, suggesting that this causes a slight increase in the barrier to the crystallisation of the β-form in comparison to the α-form. The solution populations of the self-associated OH⋯O H-bonding ‘classic carboxylic acid dimer’, present in the α- and not the β-structure, is calculated to dominate in acetonitrile, dimethyl sulfoxide, ethanol, ethyl acetate, methanol, nitromethane and water. It is observed that this classic dimer is least stable in water, compared to the other PABA crystallisation solvents, with the OH⋯N H-bonding interaction present in the β-form being the second most stable dimeric interaction. These results are discussed in terms of the crystallisability and polymorphic behaviour of the α and β forms of PABA from the afore mentioned crystallisation solvents, whilst detailing how this approach could be reproducible for a range of polymorphic crystalline materials.