Chlorine Nuclear Quadrupole Resonance Study of the Molecular and the Ionic Crystals of Phosphorus Pentachloride and the Phase Transitions in and between the Two Crystalline Modifications

The 35Cl quadrupole resonance (NQR) spectra in both the molecular (PCl5) and the ionic (PCl4+·PCl6-) solid modifications were measured and assigned, and their temperature dependences (20 to 250°K) were interpreted by a modified Bayer theory. A phase transition of higher order occurs in the ionic solid at 102.3°K which was also revealed by a preliminary heat capacity measurement: There are seven resonance lines below 102.3°K and four above it. An anisotropy in the librational motion is suggested. The molecular solid modification is metastable and transformation into the ionic solid occurs over a range of temperature. This was observed both in the quadrupole resonance spectrum and in the differential thermal analysis; the latter was used to derive the apparent activation energy for the disproportionation reaction. A phase transition occurs also in the molecular solid at 183°K. The partial ionic character of the P-Cl bonds in the two modifications was estimated from the NQR results and could account for the observed chemical shift in the nuclear magnetic resonance of 31P. The 35Cl resonance frequencies in a family of compounds PXn were correlated.