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We report the construction of a superambient temperature adiabatic heat-capacity calorimeter which has been successfully operated from 300 to 550 K. We detail results on the heat capacity of a portion of the Calorimetry Conference sample for a-Al,O,. These results are compared with those of other investigators.

Heat capacity measurements from 5 to 350°K and vapor pressure measurements on bis(benzene)chromium together with the published frequency assignments permit a correlation between the third-law and spectroscopic entropies. Neither thermal anomalies nor zero point entropy were found. The agreement is consistent with free rotation about the ring to metal bonds and Dsh symmetry for the molecuIe as proposed by Cotton. The thermodynamic functions for the crystal at 298.15"K are 53.52,54.07,26.77, and-27.30 cal. mole-r-"K-1 for the heat capacity, entropy, enthalpy function, and Gibbs function, respectively. The vapor pressure over the range 310 to 365°K is represented by logrep (mm)=27.42-5451/T-5.535 logreT, (T in OK).

The enthalpies of combustion, AH~, for crystalline [2.2]-paracyclophane (C~6H1~) and triptycene (C2oH~) have been measured by oxygen combustion calorimetry. The derived standard enthalpies of formation at 298.15 K in the crystalline state are (34.594-0.19) and (51.874-0.20) kcalth mol-1. The strain present in these molecular systems is discussed.

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