Intriguing High Z″ Cocrystals of Emtricitabine

Emtricitabine (ECB) afforded dimorphic cocrystals (Forms I, II) of benzoic acid (BA), whereas p-hydroxybenzoic acid (PHBA) and p-aminobenzoic acid (PABA) resulted in high Z″ cocrystals. Intriguingly, the Z″ of cocrystals are trends from 2 to 14 based on the manipulation of functional groups on the para position of BA (where the H atom is replaced with that of OH or NH2 group). The ECB–PABA cocrystal consists of six molecules each and two water molecules in the asymmetric unit (Z″=14) with 2D planar sheets representing the rare pharmaceutical cocrystal. The findings suggest that the increment of H bond donor(s) systematically via a suitable coformer is in correspondence with attaining high Z″ cocrystals. Further, solid state NMR spectroscopy in conjunction with single crystal X-ray diffraction is demonstrated as a significant tool to enhance the understanding of the number of symmetry independent molecules in the crystalline lattice and provide insights to the mechanistic pathways of crystallization.