Orientational Disorder in Adamantane and Adamantanecarboxylic Acid

The molecular crystals adamantane, C₁₀H₁₆, and adamantanecarboxylic acid, C₁₀H₁₅COOH, undergo order–disorder phase transitions at 208 and 250 K, respectively. Reverse Monte Carlo refinement of total neutron scattering data collected from deuterated samples immediately above these phase transitions shows that the high-temperature phases are well described by models in which the adamantyl groups are disordered over two sites. No correlation between the orientations of neighbouring molecules is observed. These results demonstrate that the intermolecular potential energy of these materials depends strongly on the orientation of the reference molecule but only very weakly on the orientations of its neighbours.