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

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 P212121, Z = 4) and a high-temperature plastic phase (Fm3̅m, 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 (P1̅, 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
(P63/mcm, Z = 6) at ca. 241 K and the highest one, cubic (Fm3̅m, 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···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.