A concealed anomaly at 117.5 K in the heat capacity of hexamethylbenzene

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Abstract
Heat capacities of solid hexamethylbenzene were measured in an adiabatic calorimeter between 3 and 300 K with particular attention to the phase transition at 117 K between phase III and phase II. The transition is very likely to be of first order with the temperature 117.5±0.1 K, the heat of the transition 0.99±0.02 kJ mol−1, and the entropy of transition 8.3±0.2 J K−1 mol−1. The reason why the transition has been referred to as a λ transition is because there is an additional thermal anomaly concealed behind the transition, which has a maximum as large as 50 J K−1 mol−1 at 128 K. This anomaly, or hump in the heat capacity curve, was interpreted in terms of rotatory oscillation/hindered rotation of the methyl groups in a potential barrier of the shape V = V3 cos 3ϑ+V6 cos 6ϑ+V9 cos 9ϑ with V3 = 4×10−21 J, V6 = 1×10−21 J, and V9 = 3×10−21 J. The effect of the transition on the hump is merely to cause a small jump and the hump is otherwise continuous beginning in phase III through phase II. A term, excitation index, is proposed to characterize a first order portion of a phase transition.

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