Pressure-Induced High-to-Low Z' Phase Transition of a Conformationally Disordered Molecular Crystal.

Conformational disorder in the structure of monoclinic phase α of 6-chloro-4,4,5,7-tetramethyldihydrocumarin (C12H15ClO2), with four symmetry-independent molecules (Z' = 4) is eliminated either by cooling below 200 K at 0.1 MPa or by compression to above 0.64 GPa at 296 K. However, high-pressure conditions are essential for transforming this compound to triclinic phase β, structurally related to phase α, but with two ordered molecules in the asymmetric unit (Z' = 2). The isothermal compression triggers the α-to-β transition at 1.65 GPa, whereas 0.7 GPa suffices for obtaining phase β by the isochoric recrystallization. In phase α, two pairs of independent molecules are differentiated by the conformational disorder, but its elimination at 200 K does not trigger the phase transition, even at 100 K. [ABSTRACT FROM AUTHOR]