Thermodynamic properties and the plastic crystal state of 2-methyl-2-adamantanol

The heat capacity investigation of 2-methyl-2-adamantanol in the condensed state was carried out in a vacuum adiabatic calorimeter between 5 and 310 K and in a scanning calorimeter of the heat bridge type between 300 and 530 K. It was found that 2-methyl-2-adamantanol has a phase transition into the “plastic” crystal state at 367.5 K. The thermodynamic characteristics of solid-to-solid phase transition and fusion (Tfus=488.8 K) were obtained. According to the X-ray crystallography, the crII had the rhombic lattice of the space group Pcc2 and the crI had the cubic face-centered (fcc) lattice of the space group F4̄3c. The lattice parameters were calculated. The entropy, the number of non-equivalent orientations and the energy difference between the “plastic” and the “basic” orientations of the molecules in the “plastic” crystal of 2-methyl-2-adamantanol were determined from the heat capacity. The contributions to the heat capacity and the entropy of the “plastic” crystal caused by the possible existence of the molecular dimers due to the hydrogen bonding were considered. The enthalpy of the hydrogen bond was determined from the shifts of the OH valence vibration bands in the IR spectra of 2-methyl-2-adamantanol.