Metastable beta-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 p olymorphic. It is well known that benzophenone crystallizes in a stable ort horhombic alpha-form (m.p. 321 K) with space group P2(1)2(1)2(1) and a = 10 .28, b = 12.12, c = 7.99 Angstrom, [Girdwood (1998). Ph.D. thesis. Strathcl yde University, Glasgow, Scotland]. Here we report two separate structure d eterminations of the metastable beta-form (m.p. 297-299 K). Crystalline mat erial of the metastable polymorph was obtained from a melt supercooled to s imilar to 243 K. The structure was determined from X-ray powder diffraction data by employing a novel, computational systematic search procedure to id entify trial packing arrangements for subsequent refinement. Unit-cell and space-group information, determined from indexing the powder diffraction da ta, 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 = 8.1 5, c = 16.33 Angstrom, beta = 112.91 degrees (at 223 K). The structures der ived from the individual techniques are qualitatively the same. They are co mpared both with each other and with the stable polymorph and other benzoph enone derivatives.