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 <i>P</i>2₁2₁2₁, <i>Z</i> = 4) and a high-temperature plastic phase (<i>Fm</i>3̅<i>m</i>, <i>Z</i> = 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 (<i>P1̅</i>, <i>Z</i> = 2), which transforms to a monoclinic phase (<i>C</i>2/<i>c</i>, <i>Z</i> = 8) at 224.4 ± 1.0 K, both phases being ordered. Two high-temperature orientationally disordered are found for this compound, one hexagonal (<i>P6</i>₃<i>/mcm</i>, <i>Z</i> = 6) at ca. 241 K and the highest one, cubic (<i>Fm</i>3̅<i>m</i>, <i>Z</i> = 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···Br as well as C–Br···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