Molecular association in low-temperature and high-pressure polymorphs of 1,1,1,2-tetrachloroethane

Abstract

Interactions and aggregation of 1,1,1,2-tetrachloroethane molecules, Cl₃CCH₂Cl, have been investigated at low temperature and high pressure. Isobaric and isochoric crystallizations led to two polymorphs, characterized by single-crystal X-ray diffraction. The low-temperature polymorph α is monoclinic, space group C2/c, with molecules orientationally disordered in two sites at the temperature independent 70:30 rate. In isothermal conditions (295 K) 1,1,1,2-tetrachloroethane freezes at 0.73 GPa. The high-pressure polymorph β is monoclinic, space group P2₁/c, with the molecules fully ordered. The molecular aggregation at varied thermodynamic conditions results from the interplay of halogen interactions, molecular symmetry and packing efficiency of the molecules. The intermolecular interactions in 1,1,1,2- and 1,1,2,2-tetrachloroethane isomers have been correlated with their melting points, lower by ca. 25 K for the former compound.