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 β (space group <i>P</i>2₁2₁2₁ with <i>Z</i> = 8 and <i>Z</i>′ = 2), formed by molecules with one of the two <i>gauche</i> conformations. At high pressure, the stable polymorph is known to be monoclinic (space group <i>P</i>2₁/<i>c</i>, with <i>Z</i> = 2 and <i>Z</i>′ = 0.5), phase α, in which only the <i>trans</i> conformer appears. In this work, we demonstrate the existence of a normal pressure metastable polymorph, phase γ, for which the two <i>gauche</i> conformers show up in the asymmetric unit of a monoclinic (space group <i>P</i>2₁/<i>c</i> with <i>Z</i> = 8 and <i>Z</i>′ = 2) structure. The new phase γ is obtained by recrystallization upon heating the glass obtained after quench of the melt. It displays shorter type II Cl···Cl contacts than the high-pressure phase α due to attractive and directional interactions