Density, freezing and molecular aggregation in pyridazine, pyridine and benzene

Abstract

Molecular aggregation of pyridazine (C₄H₄N₂) and pyridine (C₅H₅N) has been compared with that of benzene (C₆H₆) in its phases I and II. The crystal structures of pyridazine and pyridine, in situ crystallized in a diamond-anvil cell, have been determined by single-crystal X-ray diffraction at 0.27 and 0.61 GPa (C₄H₄N₂) and 1.23 GPa (C₅H₅N). The isochoric and isothermal freezing of pyridazine yielded monoclinic crystals, space-group P2₁/n, of the same symmetry as those obtained by isobaric freezing at 0.1 MPa below 100 K. At 295 K pyridine froze at 1.23 GPa in orthorhombic space-group Pna2₁. The compressibility measurement of both compounds have been performed at 295 K up to ca. 2 GPa and confirmed their freezing pressure of 0.19 GPa for pyridazine and 0.53 GPa for pyridine. The interactions governing the molecular arrangement in the series of pyridazine, pyridine and benzene structures, gradually change from C–H⋯N to C–H⋯π hydrogen bonds. High pressure also favours the C–H⋯N interactions over the C–H⋯π bonds in pyridine, but benzene remains more stable than pyridazine and pyridine. The phase diagram of pyridine has been outlined up to 2.0 GPa. A pyridine–methanol 3 : 1 co-crystal has been obtained at 1.80 GPa/295 K.