Isothermal and isochoric crystallization of highly hygroscopic pyridine N-oxide of aqueous solution.

Highly hygroscopic pyridine N-oxide, C5H5NO, dissolves in water absorbed from atmospheric air, but it crystallizes in the neat form of the aqueous solution under high pressure. The crystals grown at high-pressure isochoric conditions are of the same phase as that obtained from anhydrous crystallization at ambient pressure. This feature can be employed for retrieving compounds highly soluble in water from their aqueous solutions. The crystal structure is strongly stabilized by CH...O contacts. The crystal compression and thermal expansion as well as three shortest H...O distances comply with the inverse-relationship rule of pressure and temperature changes.