Phase Transition in Hydrogen-Bonded 1‑Adamantane-methanol

The polymorphism of 1-adamantane-methanol C₁₁H₁₈O has been investigated by differential thermal analysis and single-crystal and powder X-ray diffraction. Below the melting temperature (389.5 ± 0.4 K), this compound exhibits an orthorhombic phase (phase I, Pnnm, Z = 12, Z′ = 1.5). The melting enthalpy was determined to be 20.5 ± 0.4 kJ mol^–1, i.e., with an entropy change of (6.34 ± 0.13)­R, which is much higher than the quoted value from Timmermans for the melting orientationally disordered phases (2.5R), thus supporting the orientationally ordered character of phase I. This orthorhombic phase I exhibits a statistical disorder of the hydrogen atom related to the oxygen atom, due to the position of one independent molecule on the mirror. At ca. 272 K, phase I transforms continuously through an order–disorder transition to a low-temperature monoclinic phase II (P2₁/n, Z = 12, Z′ = 3). The monoclinic and orthorhombic phases are related by a group–subgroup relationship, which perfectly agrees with the continuous character of the II to I transition. Moreover, by a convenient choice of an order parameter related to the continuous tilt of the c-axis, the critical exponent for this transition is found to be close to the theoretical prediction of the three-dimensional Ising model (with a critical exponent of ca. 0.27).