Interphase Transitions and Thermophysics of ortho-and meta-Carboranes

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Abstract
The heat capacity of o-carborane was examined by adiabatic calorimetry from 5 to 605 K (i.e., into the liquid phase) because of its higher volatility, the m-carborane was measured only through 350 K. The above data were, however, supplemented with vapor pressure determinations and dsc surveys as well as by dsc data on the p-carborane. Both o and m-forms reveal two phase transitions associated with the onset of enhanced molecular freedom and consequent plastically crystalline behavior. Entropies of the two solid-solid transitions and melting for o-carborane are 1.72 (158.2 K), 3.38 (273.6 K), and 5.18 (569.5 K) cal/(mol K), respectively. Corresponding values for the m-compound are: 3.00 (162.5 K), 3.71 (282.4 K) and 5.37 (545.8 K). For the m-form the experimental values are in good accord with the anticipated Guthrie-McCullough model entropy increment of R ln(5 × 6); those for the o-form, R ln(3 × 5), can be harmonized with this mode of interpretation only by postulating the “missing” R ln 2 entropy as a zero point entropy. This would be consistent with the 298.15 K values of the standard entropies for these compounds-observed as 50.25 and 51.26 cal/(mol K). The larger dipole moment of the o-carborane, 4.53 D, compared with 2.85 D for the m-form may well occasion the difference in third-law behavior.