Conformational Polymorphism: The Missing Phase of 1,1,2,2-Tetrachloroethane (Cl2HC-CHCl2)

Halogenoethane derivatives are known to exhibit different polymorphs involving a different translational, orientational, and conformational order. The 1,1,2,2-tetrachloroethane (Cl2HC-CHCl2) exhibits a normal pressure orthorhombic phase beta (space group P2(1)2(1)2(1) with Z = 8 and Z' = 2), formed by molecules with one of the two gauche conformations. At high pressure, the stable polymorph is known to be monoclinic (space group P2(1)/c, with Z = 2 and Z' = 0.5), phase alpha, in which only the trans conformer appears. In this work, we demonstrate the existence of a normal pressure metastable polymorph, phase gamma, for which the two gauche conformers show up in the asymmetric unit of a monoclinic (space group P2(1)/c with Z = 8 and Z' = 2) structure. The new phase gamma is obtained by recrystallization upon heating the glass obtained after quench of the melt. It displays shorter type II Cl center dot center dot center dot Cl contacts than the high-pressure phase alpha due to attractive and directional interactions.