Ab initio structure determination of triphenyl phosphite by powder synchrotron X-ray diffraction

The crystal structure of the glass-forming molecular liquid triphenyl phosphite [TPP, P(OC6H5)3, six torsional degrees of freedom] has been solved ab initio at 110 K from powder synchrotron X-ray diffraction data by real-space methods (simulated annealing) followed by rigid-body Rietveld refinements. The symmetry is trigonal, with a rhombohedral lattice, space group R\bar{3}. The associated hexagonal cell, which is a sixfold multiple of the previously published less-symmetric monoclinic cell [Hedoux et al. (1999). Phys. Rev. B, 60, 9390–9395], is unusually large [V = 7075.7 (4) A3, Z = 18] and displays a noteworthy platelet-like shape [a = 37.766 (1) and c = 5.7286 (2) A]. The TPP molecule does not exhibit the ideal C3 symmetric propeller shape, its conformation being on the contrary almost mirror-symmetric, with the pseudo-mirror plane passing through the P—O1 bond and two carbon atoms in para position with respect to the O1 atom. The hitherto unknown topological features of crystalline TPP, including unusual intermolecular weak C—H⋯O hydrogen-bonding networks, are presented and discussed in the scope of the `glacial state' problem.