Phase transitions in 2-bromonaphthalene. The structure of the low temperature phase from X-ray powder diffraction

Abstract

Abstract The crystal structure of the low temperature stable form of 2-bromonaphthalene has been solved ab initio from laboratory X-ray powder diffraction data. Crystal data: a = 9.572(2), b = 9.599(2), c = 10.307(1) A , α = 100.75(1), β = 109.04(1), γ = 101.76(1) ° , P 1 space group, C 10 H 7 Br, Z = 4, 298 K. 2-bromonaphthalene occurs in three polymorphic varieties. While the crystal structures of the high temperature form and of the metastable form at 25 °C were already known, the structure of the low temperature form remained unsolved due to the difficulty of growing single-crystals. In spite of its complexity (two molecules in the asymmetrical unit), the structure has been solved from powder data at moderate resolution (2.2 A) combining direct with search methods. The subsequent Rietveld refinement with the bromonaphthalene molecules as rigid groups converged to R p = 7.6% and R wp = 10.3%. The structure is almost ordered and built of molecular layers stacked along [001] with the long molecular axis normal to the layer. The orientation of the naphthalenes within the layers is nearly the same as in the structure of the unsubstituted naphthalene. The probable mechanism of the transition from metastable to low temperature form has been established.