The crystal and molecular structures of adamantanecarboxylic acid at 173 and 280 K

Abstract

The crystal structure of adamantanecarboxylic acid, tricyclo[3.3.1.13,7] decan-1-carboxylic acid C10H15COOH, has been determined by X-ray diffraction at two temperatures, 173 and 280 K. At both temperatures, the unit cells are triclinic, space group[Formula: see text], and each contains two molecules. At 173 K, a = 6.452(3), b = 6.681(2), c = 11.510(3) Å, α = 80.84(2)°, β = 87.22(3)°, γ = 74.11(3)°; V = 471.10 Å3, Dx = 1.271 g cm−3, [Formula: see text]. The structure was solved by direct-methods. The least-squares refinement, based on 1347 observed reflections converged to R = 0.0485 and Rw = 0.0415. At 280 K, a = 6.503(2), b = 6.849(2), c = 11.620(4) Å, α = 77.11(2)°, β = 85.77(2)°, γ = 76.34(2)°; V = 490.11 Å3, Dx = 1.221 g cm−3, [Formula: see text], R = 0.0874, and Rw = 0.0512 for 828 observed reflections. The low-temperature form is ordered while at room temperature the molecule is disordered. In the structure of the high-temperature phase, the adamantyl group takes two distinct orientations, in a 60:40 proportion, at about 14° from one another. The carboxylic group also has two orientations differing by about 170°. The geometry of the adamantane skeleton is not affected by the presence of the carboxylic group. The molecules form centrosymmetric dimers through hydrogen bonds of the [Formula: see text] type. The [Formula: see text] distances range between 2.616(11) and 2.752(15) Å. The order-disorder transition is interpreted using the Guthrie–McCullough approach. Keywords: adamantanecarboxylic acid, order–disorder transition, crystal structure.