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

Polymorphism of 2‑Adamantanone

The polymorphism of 2-adamantanone (C₁₀H₁₄O) has been investigated by means of X-ray diffraction and high-pressure thermal analysis. The intricate behavior of the low-temperature crystalline phases has been disentangled. The stable phase has been found to be orthorhombic (<i>Cmc2</i>₁, <i>Z</i> = 4), fully ordered, with lattice parameters <i>a</i> = 6.8884(18) Å, <i>b</i> = 10.830(3) Å, <i>c</i> = 10.658(3) Å, and <i>V</i>/<i>Z</i> = 198.8(1) ų. The metastable phase was determined to be monoclinic (<i>P</i>2₁/<i>c</i>, <i>Z</i> = 4) with lattice parameters <i>a</i> = 6.5920(17) Å, <i>b</i> = 11.118(3) Å, <i>c</i> = 12.589(3) Å, β= 118.869(11) ^o, and <i>V</i>/<i>Z</i> = 202.0(1) ų. The pressure–temperature phase diagram irrefutably shows the stability relation between both phases and, accordingly, the long-time unknown polymorphic behavior is now revealed and gives coherent physical explanation of the literature published so far

Polymorphism of 2‑Adamantanone

The polymorphism of 2-adamantanone (C₁₀H₁₄O) has been investigated by means of X-ray diffraction and high-pressure thermal analysis. The intricate behavior of the low-temperature crystalline phases has been disentangled. The stable phase has been found to be orthorhombic (<i>Cmc2</i>₁, <i>Z</i> = 4), fully ordered, with lattice parameters <i>a</i> = 6.8884(18) Å, <i>b</i> = 10.830(3) Å, <i>c</i> = 10.658(3) Å, and <i>V</i>/<i>Z</i> = 198.8(1) ų. The metastable phase was determined to be monoclinic (<i>P</i>2₁/<i>c</i>, <i>Z</i> = 4) with lattice parameters <i>a</i> = 6.5920(17) Å, <i>b</i> = 11.118(3) Å, <i>c</i> = 12.589(3) Å, β= 118.869(11) ^o, and <i>V</i>/<i>Z</i> = 202.0(1) ų. The pressure–temperature phase diagram irrefutably shows the stability relation between both phases and, accordingly, the long-time unknown polymorphic behavior is now revealed and gives coherent physical explanation of the literature published so far