. Can. J . Chcm. 61, 92 (1 983).The structure I clathrate hydrate of trimethylcne oxide (TMO), of composition 6TMO:46HZ0, has been studied by adiabatic calorimetry ovcr the temperature range 85-270 K. A thermal anomaly, with thc appearance of a classical A-transition, was observed between about 140-170 K. This anomaly was reproducible, provided precautions were taken against supercooling.No evidence for such a transition has been observed in previous studies using dynamic relaxation techniques (dielectric and nrnr relaxation). The temperature range of this anomaly is close to the melting point of a known TMO-rich eutectic, but evidence is presented which argues against interpretation of the present observations in these terms. The previous relaxation measurements indicated a transition involving mobility of thc TMO guest, centred at about 100 K. In the present work, the only evidence of this transition consisted of a much worse (order of magnitude) precision in the mcasured heat capacities in this temperature range. Thermal equilibration is known to be extremely slow in this system. The transition obscrved in the present work at higher temperatures must therefore correspond to some restructuring of the water host, and this may be connected with anomalous results obtained previously for the far infrared absorptions of water in the TMO clathrate hydrate. ['l'raduit par Ie journal] Introduction this structure I hydrate, of TMO : 7
. 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...
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