Lattice-dynamical Study of the Abnormally Short Bond-Length in trans-Stilbene Molecules in a Crystal

Kazuya SAITO and Isao IKEMOTO

Lattice-dynamics calculations have been made on crystalline trans-stilbene under an atom-atom potential approximation, using both flexible and rigid molecular models in order to clarify the existence of the molecular motion suggested for explaining the abnormal bond-length of the central C=C bond. The intramolecular twisting of the benzene rings strongly couples with the overall rotation of the molecule in some manner, including the suggested manner. The coupled modes form phonon branches and show little dispersion.
(Bull. Chem. Soc. Jpn., 69, 909-913 (1996))

The Search for a Possible Twist Phase Transition in a Biphenyl Analogue: Heat Capacity of Crystalline Tolane

Kazuya SAITO, Mayuko ASAHINA, Yasuhisa YAMAMURA and Isao IKEMOTO

Heat capacity of crystalline tolane (diphenylacetylene) has been measured by adiabatic calorimetry between 9 and 303 K in order to see possible phase transitions as in a similar compound, biphenyl. No thermal anomaly has been detected. Phonon dispersion relation of the compound is calculated using a simple model and compared with that of biphenyl. It is shown the absence of phase transition in crystalline tolane is consistent with the calculated dispersion relation. A certain height of the potential barrier at the planar conformation in the intramolecular potential for twisting motion is essential for the lattice instability of crystalline biphenyl. Some smoothed thermodynamic quantities are tabulated.
(J. Phys.: Condensed Matt., 7, 8919-8926 (1995))

Incommensurate Phase Transitions and Anomalous Lattice Heat Capacities of Biphenyl

Kazuya SAITO, Tooru ATAKE and Hideaki CHIHARA

Heat capacities of crystalline biphenyl were measured between 3 and 300 K by adiabatic calorimetry and some thermodynamic functions including the calorimetric standard entropy were tabulated. Thermodynamic properties of the successive phase transitions were determined; for the twist transition at (40.4 +- 0.2) K, deltatrsH = (5.02 +- 0.08) J mol-1 and deltatrsS = (0.129 +- 0.003) J K-1 mol-1, and for the lock-in transition at (16.8 +- 0.1) K, deltatrsH = (0.15 +- 0.02) J mol-1 and deltatrsS = (0.009 +- 0.001) J K-1 mol-1. Anomalously large heat capacities at low temperatures were analyzed based on lattice dynamics calculation, and the crossover of the low temperature heat capacities of biphenyl and p-terphenyl was attributed to the greater activity of the twisting mode in biphenyl. The relation of the bond flexbility to the incommensurability is discussed through the comparison of the low temperature heat capacities of biphenyl, p-terphenyl, and 4,4'-difluorobiphenyl.
(Bull. Chem. Soc. Jpn., 61, 679-688 (1988))

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