Polymorphism in 2-X-Adamantane Derivatives (X = Cl, Br)

Abstract

The polymorphism of two 2-X-adamantane derivatives, X = Cl, X = Br, has been studied by X-ray powder diffraction and normal- and high-pressure (up to 300 MPa) differential scanning calorimetry. 2-Br-adamantane displays a low-temperature orthorhombic phase (space group P2(1)2(1)2(1), Z = 4) and a high-temperature plastic phase (Fm (3) over barm, Z = 4) from 277.9 +/- 1.0 K to the melting point at 413.4 +/- 1.0 K. 2-Cl-adamantane presents a richer polymorphic behavior through the temperature range studied. At low temperature it displays a triclinic phase (P (1) over bar, Z = 2), which transforms to a monoclinic phase (C2/c, Z = 8) at 224.4 +/- 1.0 K, both phases being ordered. Two high-temperature orientationally disordered are found for this compound, one hexagonal (P6(3)/mcm, Z = 6) at ca. 241 K and the highest one, cubic (Fm (3) over barm, Z = 4), being stable from 244 +/- 1.0 K up to the melting point at 467.5 +/- 1.0 K. No additional phase appears due to the increase in pressure within the studied range. The intermolecular interactions are found to be weak, especially for the 2-Br-adamantane compound for which the Br center dot center dot center dot Br as well as C-Br center dot center dot center dot H distances are larger than the addition of the van der Waals radii, thus confirming the availability of this compound for building up diamondoid blocks.