Polymorphism of 2,2-Dichloropropane: Crystallographic Characterization of the Ordered and Disordered Phases

The polymorphism of 2,2-dichloropropane ((CH 3 ) 2 CCl 2 ) has been further investigated by both thermal and X-ray powder diffraction experiments. From the former the phase transitions between the different phases have been characterized at normal pressure as well as at high pressures (up to 200 MPa). From the p − T slopes of the two-phase coexistence lines, the volume variations at the transition points have been calculated and compared with those obtained by means of X-ray powder diffraction characterization. The existence of two low-temperature stable ordered phases (III and II), one high-temperature orientationally disordered phase (rhombohedral, Ib) and one additional monotropic orientationally disordered phase Ia, has been confirmed. The structure of the low-temperature ordered phase II has been determined by X-ray powder diffraction and Rietveld profile refinement as monoclinic C 2/ c, with lattice parameters a =10.6402(3) Å, b = 5.4074(2) Å, c = 10.7295(3) Å, and β = 116.274(3)° at 175.2 K. The strength of intermolecular interactions as well as the anisotropy have been analyzed by means of the thermal-expansion tensor.