Metastable β-phase of benzophenone: Independent structure determinations via X-ray powder diffraction and single crystal studies

Benzophenone was the first organic molecular material to be identified as polymorphic. It is well known that benzophenone crystallizes in a stable orthorhombic a-form (m.p. 321 K) with space group F2₁2₁2₁ and a = 10.28, b = 12.12, c = 7.99 Å, [Girdwood (1998). Ph.D. thesis. Strathclyde University, Glasgow, Scotland]. Here we report two separate structure determinations of the metastable ß-form (m.p. 297-299 K). Crystalline material of the metastable polymorph was obtained from a melt supercooled to ~243 K. The structure was determined from X-ray powder diffraction data by employing a novel, computational systematic search procedure to identify trial packing arrangements for subsequent refinement. Unit-cell and space-group information, determined from indexing the powder diffraction data, was used to define the search space. The structure was also determined from single-crystal diffraction data at room temperature and at 223 K. The metastable phase is monoclinic with space group C2/c and a = 16.22, b = 8.15, c = 16.33 Å, ß = 112.91° (at 223 K). The structures derived from the individual techniques are qualitatively the same. They are compared both with each other and with the stable polymorph and other benzophenone derivatives.