Pre-melting transitions in caged hydrocarbons, a general theory of disorder in plastic crystals

Clark, Timothy; McKervey, M. Anthony; Mackle, H.; Rooney, John J.

doi:10.1039/f19747001279

Cited by 48 publications

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“…As discussed by Westrum and Henriquez, the total transition entropy of m-carborane is close to Rϫln 30(ϭ28.3 J K Ϫ1 mol Ϫ1 ) which is expected from a statistical model considering the molecular and site symmetries, 45,46 indicating that m-carborane is orientationally ordered at the lowest temperature phase (T Ͻ170 K). If m-carborane is ordered at 0 K and the configurational states of both carboranes are the same at the high temperature phase ͑these are both quite plausible assump-tions͒, the difference between the transition entropies of mand o-carboranes (10.8 J K Ϫ1 mol Ϫ1 ϭRϫln 3.7) suggests that o-carborane has residual disorder involving about four orientations at 0 K. This means that o-carborane is not categorized into the orientational glasses ͑such as ethanol, cyclohexanol, etc.͒ in which molecular orientation is disordered as much as in liquids, but rather into the orientational glasses ͑such as RbCN and C 60 ) in which molecules are disordered among only a small number of orientations.…”

Section: Transition Entropy and Orientational Disordersupporting

confidence: 59%

Calorimetric study of plastically crystalline o- and m-carboranes

Yamamuro

Hayashi

Matsuo

et al. 2003

The Journal of Chemical Physics

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Articles you may be interested inCalorimetric and dielectric study of organic ferroelectrics, phenazine-chloranilic acid, and its bromo analog J. Chem. Phys. 130, 034503 (2009); 10.1063/1.3058589 2 H nuclear magnetic resonance study of the molecular motion in cyanoadamantane. I. Supercooled plastically crystalline phase Study of the nature of glass transitions in the plastic crystalline phases of cyclo-octanol, cycloheptanol, cyanoadamantane and cis-1,2-dimethylcyclohexane Structural signature of the configurational entropy change during the glass formation of plastic crystal cyanoadamantaneThe heat capacities of o-and m-carboranes were measured with an adiabatic calorimeter in the temperature range 5-310 K. m-carborane underwent two phase transitions at 170 K and 284 K with transition entropies of 12.70 and 15.52 J K Ϫ1 mol Ϫ1 . The total transition entropy agreed well with the theoretical value, indicating that m-carborane is ordered at 0 K. o-carborane also exhibited two phase transitions at 160 K and 275 K with transition entropies of 3.72 and 13.72 J K Ϫ1 mol Ϫ1 , and additionally a glass transition at 120 K. This glass transition does occur not in the supercooled intermediate-temperature phase but in the low-temperature phase of o-carborane. The high-and intermediate-temperature phases of o-carborane could not be quenched at the maximum cooling rate ͑10 K min Ϫ1 ͒ of this experiment. The total transition entropy of o-carborane was smaller than that of m-carborane by 10.8 J K Ϫ1 mol Ϫ1 , suggesting that o-carborane has residual orientational disorder at 0 K. The enthalpy relaxation was examined by the temperature jump method around the glass transition of o-carborane. The results were reproduced well by the KWW function. The calorimetric relaxation time and nonexponential parameter agreed with dielectric ones at the glass transition temperature. This indicates that the glass transition of o-carborane is due to the freezing of molecular reorientation.

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Section: Transition Entropy and Orientational Disordersupporting

confidence: 59%

Calorimetric study of plastically crystalline o- and m-carboranes

Yamamuro

Hayashi

Matsuo

et al. 2003

The Journal of Chemical Physics

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“…While a 62-fold disordering process is unrealistic, the 1 .Sfold or 614-fold disordering at the I1 + I transition could be due to a transition from a lattice of cubic to one of hexagonal symmetry. An alternative interpretation of the I11 + I1 transition can be made if the approach of Clarke et al (14,15) is taken, in which a contribution of approximately 10.3 J K-' mol-' is subtracted from the entropy of transition to account for the slackening of the lattice. This approach would produce a value of R In 18 for the configurational entropy, and hence an 18-fold disordering at the transition.…”

Section: Resultsmentioning

confidence: 99%

Phase transitions in solid cycloheptene

Haines¹,

Gilson²

1990

Can. J. Chem.

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. Can. J. Chem. 68, 604 (1990). The phase transition behaviour of cycloheptene has been investigated by differential scanning calorimetry, proton spin-lattice relaxation, and vibrational spectroscopy (infrared and Raman). Two solid-solid phase transitions were observed, at 154 and 210K, with transition enthalpies and entropies of 5.28 and 0.71 kJ mol-I and 34.3 and 3.4 J K-', respectively. Cycloheptene melted at 2 17 K with an entropy of melting of 4.5 J K-' mol-I. The bands in the vibrational spectra of the two high temperature phases were broad and featureless, characteristic of highly disordered phases. The presence of other conformers, in addition to the chair form, was indicated from bands in the spectra. The ring inversion mode was highly phase dependent and exhibited soft mode type behaviour prior to the transition from the low temperature phase. The low frequency Raman spectra (external modes) of these phases indicated that the molecules are undergoing isotropic reorientation. In the low temperature phase, the vibrational bands were narrow; the splitting of the fundamentals into two components and the presence of nine external modes are consistent with unit cell symmetry of either C2 or C, with two molecules per primitive unit cell. A glassy state can be produced from the intermediate phase and the vibrational spectra were very similar to those of the high temperature phases, indicating that static disorder was present. The barriers to reorientation, as obtained from proton spin-lattice relaxation measurements, are 9.0 kJ mol-' in both the high temperature phases, and 15.4 kJ mol-I in the low temperature, ordered phase.

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“…This is an unreasonably large number of distinguishable orientations. Clark and coworkers (15,16) have examined the entropies of transition for several cage hydrocarbons and proposed an empirical equation involving an excess entropy, which depends upon the temperature range of the disordered phase, eq. [2].…”

Section: Introductionmentioning

confidence: 99%

Phase transitions in adamantane derivatives: 2-chloroadamantane

Paroli¹,

Kawai²,

Butler³

et al. 1988

Can. J. Chem.

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Thispaper is dedicated to Professor Charles A. McDowell on the occasion of his seventieth birthdayRALPH M. PAROLI, NANCY T. KAWAI, IAN S. BUTLER, and DENIS F. R. GILSON. Can. J. Chem. 66, 1973 (1988. The phase transition behaviour of 2-chloroadamantane, 2-CloHlsC1, has been investigated by differential scanning calorimetry (DSC), and FT-IR and Raman spectroscopy. Two transitions were detected by both DSC and vibrational spectroscopy at 23 1 and 178 K, on cooling, and at 242 and 227 K, on heating. The measuredenthalpies were 8.3 kJ mol-' for the first transition (phase I + phase 11), and 0.47 kJ mol-' for the second (phase I1 + phase 111). The entropies were 35 and 2.3 J K-' mol-I, respectively. These are similar to those observed for other 2-substituted adamantanes, but significantly different from those for 1 -substituted derivatives. The large hystereses observed for the two transitions are independent of the DSC scanning rate and are characteristic of first-order phase transitions. The dramatic differences observed in the vibrational spectra of phases I and I1 provide clear evidence of an order-disorder transition at about 235 K.

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Pre-melting transitions in caged hydrocarbons, a general theory of disorder in plastic crystals

Cited by 48 publications

References 0 publications

Calorimetric study of plastically crystalline o- and m-carboranes

Calorimetric study of plastically crystalline o- and m-carboranes

Phase transitions in solid cycloheptene

Phase transitions in adamantane derivatives: 2-chloroadamantane

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