Crystal structures and glassy phase transition behavior of cyclohexene

The complex phase behavior of cyclohexene, C ₆H ₁₀, has been characterized between 2 K and its melting point at 170 K using complementary high-resolution neutron powder diffraction and single-crystal X-ray diffraction techniques, and the crystal structures of the three known ambient-pressure phases have been determined. Phase II is the sole orientationally ordered structure of cyclohexene, crystallizing in the triclinic space group P1̄. Phase I and metastable phase III may be characterized by the mode of orientational disorder corresponding to free uniaxial rotation and ring inversion of the molecules, respectively. Phase I is cubic with space group Pa3̄, and the molecule is located on a 3-fold axis with Z′ = 1/3. Phase III is monoclinic with Z′ = 2 and space group P2 ₁/c, but with pseudocubic character inherited from the phase I structure. In this phase, only one of the molecules in the asymmetric unit is disordered and exhibits ring inversion which freezes at the glass transition temperature around 80 K.