%Variable-Temperature Studies of Order/Disorder Transitions in the Thiourea Pyridinium Halide Crystals by XRD and Solid-State ²H NMR

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.

Discovery of an intermediate phase in bis-thiourea pyridinium chloride inclusion compound

“…The phase transition from the HT Cmcm to INT Cmc2 1 with the a axis tripled ͑a =3a HT ͒ is very similar to the ones observed for the bromide compounds and reported by Prout et al 17 Since all of the T 2 ͑PyH͒Cl crystals undergo these phase changes with keeping very good crystallinity, it has been possible to determine the structures of the phases above and below the transition temperatures. Full data sets were collected at 265, 208, and 150 K to check structural reorganization associated with the two-step transition ͑the CCDC Reference No.…”

“…These complexes are ionic-molecular systems held by van der Waals and Coulomb interactions as well as hydrogen bonds. Hitherto, four such systems with pyridinium chloride, bromide, iodide, and nitrate have been described ͓hereafter denoted as T 2 ͑PyH͒Cl, T 2 ͑PyH͒Br, T 2 ͑PyH͒I, and T 2 ͑PyH͒NO 3 , respectively͔ [17][18][19][20][21][22][23][24][25] and very recently the complex with thiazolium bromide was discovered. 26 The thiourea molecules connected by hydrogen bonds form ribbons, and four such ribbons make a channel of a rectangular ͑or close to rectangular͒ shape containing pyridinium cations.…”

“…27 Although the room-temperature structure of T 2 ͑PyH͒Br was already determined in 1972 by Truter and Vickery 28 the complex variable-temperature single-crystal x-ray diffraction experiment for all three halides has been performed by Prout et al 17 The study performed in a wide temperature range has led to the identification of the different phases of these compounds: two phases in T 2 ͑PyH͒Cl and T 2 ͑PyH͒I and three phases in T 2 ͑PyH͒Br. The authors did not specify the temperatures of the phase transitions giving only rough estimates.…”

“…17. The structure of the INT phase has been determined and the temperature changes in the unit cell parameters and intensity of selected reflections characterizing the nature of the symmetry breaking associa͒ Electronic mail: apajzder@amu.edu.pl.…”

“…From previous x-ray diffraction data, it was reported that a phase transition occurs between the HT phase ͑Cmcm͒ and the LT phase ͑Pbca͒, 17 This was detected by the appearance of numerous Bragg reflections indexed ͑hkl͒ with h =2n + 1 or 2n +2 ͑n is an integer͒ in the reciprocal lattice corresponding to 3a HT . Figure 2͑a͒ shows the intensity dependence of such Bragg reflections, which indicates a second order phase transition between the HT and INT phases around 237 K. Small anomalies are observed at the same temperature on the slope of the temperature dependence of the lattice parameters given in the HT lattice ͓Figs.…”