Hydrate smaller than the anhydrate†
Anhydrous 5,6-dimethylbenzimidazole (dMBzIm) crystals are obtained from an aqueous solution at ambient pressure, but above 0.25 GPa the hemihydrate dMBzIm·½H2O is formed. It can be stored for over a year in open vials. Counter-intuitively, the volume of dMBzIm·½H2O is smaller than that of the anhydrate. Ambient and high-pressure structures of dMBzIm and dMBzIm·½H2O crystals grown in situ in a diamond-anvil cell have been determined by X-ray diffraction methods. The isostructural phase transition observed in dMBzIm at 0.25 GPa coincides with the changed preference to hemihydrate crystallization. The significantly different ambient and high-pressure hydration preferences of imidazole and benzimidazole derivatives have been explained by the interplay of interactions and the efficiency of crystal packing measured by the size and volume of voids.