No abstract

No abstract

No abstract

No abstract

Variable-temperature single-crystal X-ray experiments have been performed on the thiourea pyridinium chloride, bromide, and iodide inclusion compounds. Two phases were found for the chloride, three for the bromide, and two for the iodide. As previously reported for the bromide, at room-temperature all three halides were found to form orthorhombic crystals in space group Cmcm. For each compound, the pyridinium cation is disordered in the square cross section thiourea channels. On cooling, the chloride undergoes a phase change between 200 and 230 K to give an orthorhombic form in Pbca with the a axis doubled and the pyridinium ions partially ordered. The bromide undergoes a phase change between 170 and 165 K to a partially ordered form in space group Cmc21 with the a axis tripled. A second phase change occurs between 140 and 150 K to a fully ordered phase in space group P21 cn with cell dimensions approximately the same as the room-temperature form. The iodide undergoes a single-phase transition at 120−130 K to a fully ordered form isomorphous with the low-temperature form of the bromide. The phase transitions have also been studied by VT 2H NMR of the perdeuteriopyridinium salts. These data have been correlated with the XRD data and with models for the motion of the pyridinium cation.