Entropy changes and structural implications for crystalline phases of pyrazine

Abstract

. Can. J. Chem. 57,3056 (1979). Molar heat capacities of crystalline pyrazine have been measured by adiabatic calorimetry in the range 20-40°C and interpreted to show that in the crystal structures of phases I1 and I11 half the molecules must be disordered. Together with previous X-ray studies, this allows possible structures for phase I1 and phase 111 to be deduced. Of the eight molecules in the phase 111 unit cell, four are disordered over two sites so that the point symmetry is effectively mmm; the remaining four molecules have 2/m symmetry and are not disordered. This structure is consistent with the available spectroscopic evidence. It is likely that the phase I1 structure is closely related to the phase I11 structure, for example by the molecules with 2/m symmetry adopting a slightly different orientation.ROBERT K. BOYD, JOHN COMPER et GEORGE FERGUSON. Can. J. Chem. 57,3056(1979). On a mesure les capacites calorifiques molaires de la pyrazine cristallisee, a l'aide d'un calorimetre adiabatique dans un intervalle de temperature de 20-40°C. Les rtsultats sont interpretes dans le but de montrer que dans les structures cristallines des phases I1 et I11 la moitie des molecules doivent Ctre desordonnees. De concert avec les etudes anterieures de rayons-X, ceci permet de deduire les structures possibles des phases I1 et 111. Des huit moltcules dans la maille elementaire de la phase 111, quatre sont desordonnees sur deux sites de telle sorte que le point de symetrie soit effectivement mmm. Les quatre molCcules restantes ont une symetrie 2/n2 et ne sont pas desordonnees. Cette structure est compatible avec les preuves spectroscopiques disponibles. I1 est probable que la structure de la phase I1 soit reliee de pres a celle de la phase 111, par exen~ple par l'intermediaire des moltcules a symetrie 2/m adoptant une orientation 16gerement differente.[Traduit par le journal]The nature of the phase transitions in crystalline pyrazine has been the subject of several investigations incorporating a wide range of techniques; thus, infrared (1) and Raman (2) studies both detected a phase transition at about 29°C. Similarly, an nqr study (3) of the 14N quadrupole coupling constants in pyrazine yielded evidence of a phase transition near 27"C, and also of a second transition near 35°C. Measurements of proton spin-lattice relaxation times (4) ( T I and TID) in crystalline pyrazine, on the other hand, resulted in no indication of any discontinuity in the appropriate temperature range being detected.The solid state structure of phase I of pyrazine has been fully defined by X-ray methods at 20°C (5) and at -89°C (6) and more recently (7) an investigation of the phase I-II (28.5"C) and 11-111 (36.5"C) transitions of pyrazinel has been reported in which such techniques as X-ray diffraction, calorimetry, and lowfrequency infrared and Raman spectroscopy were used.lThe three distinct phases were labelled I, 11, 111, in order of increasing temperature. This is contrary to the usual convention, which uses "Phase I" to describe the highest...

Measurements of Vapor Pressure, Heat Capacity, and Density along the Saturation Line for ε-Caprolactam, Pyrazine, 1,2-Propanediol, Triethylene Glycol, Phenyl Acetylene, and Diphenyl Acetylene

“…The dashed line represents the deviations from the equation listed in Table 2 detailing experimental measurements of the critical properties or saturation liquid-phase densities over a range of temperature for pyrazine. In addition to a paper reporting saturated vapor pressures, 35 the only detailed thermophysical properties reported relate to the measurement of the enthalpies associated with phase transitions in pyrazine [36][37][38] and to its energy of combustion. 39 With the exception of one listed point (332.44 K/13.79 kPa), the vapor pressures obtained by Sakoguchi et al 35 are in good agreement ((1%) with values obtained using eq 2 and the parameters listed in Table 5.…”

“…The phase transition measurements of Schettino et al, 36 Bougeard et al, 37 and Boyd et al 38 all agree on a transition at 300.65 K and point to a second one at 310 K. Boyd et al 38 measured values of 976 J‚mol -1 and 963 J‚mol -1 for the transition at 300.65 K which can be compared with the value ∆ cII cI H°m(300.5 K) ) (0.9 ( 0.1) kJ mol -1 obtained in this research. The transition at 310 K was shown by Boyd et al to have a small excess energy, (60 ( 10) J‚mol -1 .…”

“…A search of the literature through June 1997 failed to find any papers detailing experimental measurements of the critical properties or saturation liquid-phase densities over a range of temperature for pyrazine. In addition to a paper reporting saturated vapor pressures, the only detailed thermophysical properties reported relate to the measurement of the enthalpies associated with phase transitions in pyrazine − and to its energy of combustion …”

Room-temperature structural phase transition in the quasi-2D spin- 12 Heisenberg antiferromagnet Cu(pz)2(ClO4

“…In the covered temperature range (up to 320 K), no other peaks or anomalies were detected. Earlier NMR studies of pure pyrazine [19,20] reported two anomalies at similar temperatures (301.5 K and 309 K, respectively), which were interpreted as partial order-to-disorder transitions. This similarity poses the question of whether this structural transition is also a feature of other molecular-copper complexes containing stacked pyrazine rings, such as the one-dimensional chains in Cu(pz)(NO 3 ) 2 [21] or the twodimensional Cu(pz) 2 X 2 family [2] (here X indicates a counterion, e.g., ClO − 4 , BF − 4 , and PF − 6 ).…”