The low-temperature heat capacities, phase transitions and thermodynamic properties of 1,3-dimethyladamantane and 1-ethyladamantane

The heat capacity of the crystalline and liquid phases of 1,3-dimethyladamantane (1,3-DMA) and 1-ethyladamantane (1-EA) have been measured by vacuum adiabatic calorimetry over the temperature range (8 to 373) K. 1,3-DMA and 1-EA were found to exist in two and one crystalline phases, respectively. The temperatures, enthalpies and entropies of solid-to-solid transition and fusions have been determined. The ΔfusS is less than the gas constant for 1,3-dimethyladamantane, indicates that this compound, as well as 1,3,5-trimethyladamantane (1,3,5-TMA) studied previously, forms a plastic crystalline phase before being fused. The temperature dependencies of the lattice modes were determined with Raman spectroscopy for all three substances. The thermodynamic functions obtained calorimetrically for 1,3-DMA, 1-EA and 1,3,5-TMA were corroborated through adequate accord between the calorimetric and spectroscopically determined entropy.