Spectroscopic Study of the Phase Transition in Crystalline Adamantane

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Abstract
The infrared spectrum of crystalline adamantane (tricyclo‐3,3,1,1‐decane, C10H16) has been studied at temperatures below and above the first‐order phase transition at 208°K and as a function of temperature through the transition. Splittings of the vibrational transitions and appearance of gas‐phase‐forbidden transitions below 208°K are in agreement with theoretical predictions. A hysteresis is observed in the spectroscopic characteristics over a range of a few degrees around the heat‐capacity peak occurring at the transition temperature. Calculations of lattice energy, frequency splittings, and barriers to molecular rotation have been carried out using central force functions between nonbonded atoms derived from physical properties of a number of crystals. The splittings, heat of sublimation, transition energy, and crystal structural parameters are in semiquantitative agreement with experiment. The vapor pressure of adamantane in the temperature range 312–366°K has been measured, and the heat of sublimation has been determined.