Polymorphism in cyclohexanol

Abstract

The crystal structures and phase behaviour of phase II and the metastable phases III′ and III of cyclohexanol, C6H11OH, have been determined using high-resolution neutron powder, synchrotron X-ray powder and single-crystal X-ray diffraction techniques. Cyclohexanol-II is formed by a transition from the plastic phase I cubic structure at 265 K and crystallizes in a tetragonal structure, space group P\bar 42_1c (Z′ = 1), in which the molecules are arranged in a hydrogen-bonded tetrameric ring motif. The structures of phases III′ and III are monoclinic, space groups P21/c (Z′ = 3) and Pc (Z′ = 2), respectively, and are characterized by the formation of hydrogen-bonded molecular chains with a threefold-helical and wave-like nature, respectively. Phase III crystallizes at 195 K from a sample of phase I that is supercooled to ca 100 K. Alternatively, phase III may be grown via phase III′, the latter transforming from supercooled phase I at ca 200 K. Phase III′ is particularly unstable and is metastable with respect to both I and II. Its growth is realised only under very restricted conditions, thus making its characterization especially challenging. The cyclohexanol molecules adopt a chair conformation in all three phases with the hydroxyl groups in an equatorial orientation. No evidence was found indicating hydroxyl groups adopting an axial orientation, contrary to the majority of spectroscopic literature on solid-state cyclohexanol; however, the H atom of the equatorial OH groups is found to adopt both in-plane and out-of-plane orientations.