ORIENTATIONAL DISORDER IN PHENANTHRENE - STRUCTURE DETERMINATION AT 248-K, 295-K, 339-K AND 344-K

Phenanthrene, C14H10, M(r) = 178.2, m.p. 374 K, shows a phase transition around 333-343 K. The structure is ordered at 295 K with a = 8.441 (2), b = 6.140 (1), c = 9.438 (1) angstrom, beta = 97.96 (1)-degrees, V = 484.4 (1) angstrom-3, monoclinic, P2(1), Z = 2, D(x) = 1.221 Mg m-3, lambda-(Mo K-alpha) = 0.71073-angstrom, mu = 0.064 mm-1, F(000) = 188, R = 0.043 for 726 observed reflections. The high-temperature phase is orientationally disordered with the second possible position of the molecule related to the first by a center of symmetry. Two intensity data sets at 339 and 344 K were used in least-squares refinement. The occupancy of the second position was refined to 0.158 (5) and 0.446 (3) at 339 and 344 K, respectively. It is possible to quench the high-temperature phase by rapid cooling from 344 to below 260 K. The occupancy of the second position in the quenched phase was refined to 0.389 (3).