Vapour Induced Crystalline Transformation Investigated by ab initio Powder X-ray Diffraction Analysis

A new co-crystal (phase H) of 5-methyl-2-pyridone (5MP) and trimesic acid (TMA) has been prepared by grinding a methanol solvate co-crystal (phase M) of 5MP and TMA under ambient conditions. Powder X-ray diffraction indicates that the new co-crystal phase H is structurally different from the methanol solvate co-crystal (phase M) and an unsolvated co-crystal (phase U) of 5MP and TMA reported previously. In the present work, the crystal structure of the new phase H has been determined directly from powder X-ray diffraction data, allowing insights to be gained regarding the mechanism of the transformation from M to H. The structural analysis reveals that phase H is a hydrate of 5MP and TMA, and thus the transformation from M to H is a solvent exchange process. The rate of this process is shown to be accelerated significantly by grinding. Further vapour induced transformations have been investigated for phases M, H, and U, and insights regarding transformation mechanisms have been established from consideration of the crystal structures.