research papers 486 Kutzke et al.  -phase of benzophenone Acta Cryst. (2000). B56, 486±496 Acta Crystallographica Section B Structural Science ISSN 0108-7681 Metastable b -phase of benzophenone: independent structure determinations via X-ray powder diffraction and single crystal studies Hartmut Kutzke, a Helmut Klapper, a Robert B. Hammond b * and Kevin J. Roberts b a Mineralogisch-Petrologisches Institut, Univer- sita Èt Bonn, Poppelsdorfer Schloss, D-53115 Bonn, Germany, and b Centre for Molecular and Interface Engineering, Department of Mechan- ical and Chemical Engineering, Heriot-Watt University, Edinburgh EH14 4AS, Scotland Correspondence e-mail: r.b.hammond@hw.ac.uk # 2000 International Union of Crystallography Printed in Great Britain ± all rights reserved Benzophenone was the ®rst organic molecular material to be identi®ed as polymorphic. It is well known that benzophenone crystallizes in a stable orthorhombic -form (m.p. 321 K) with space group P2 1 2 1 2 1 and a = 10.28, b = 12.12, c = 7.99 A Ê , [Girdwood (1998). Ph.D. thesis. Strathclyde University, Glasgow, Scotland]. Here we report two separate structure determinations of the metastable -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 proce- dure to identify trial packing arrangements for subsequent re®nement. Unit-cell and space-group information, deter- mined from indexing the powder diffraction data, was used to de®ne 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 A Ê ,  = 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. Received 25 February 1999 Accepted 6 January 2000 1. Introduction Benzophenone crystallizes in two polymorphic forms with melting points 321 K (stable phase) and 297±299 K (meta- stable phase; Groth, 1919). The structure of the stable phase was ®rst determined by Vul & Lobanova (1967), Lobanova (1968), Fleischer et al. (1968) and Girdwood (1998). The structure is orthorhombic with space group P2 1 2 1 2 1 and cell parameters a = 10.28, b = 12.12, c = 7.99 A Ê (Girdwood, 1998). The existence of a monoclinic metastable phase was recog- nized as early as 1871 and was the object of many early crystallization studies. However, no structural data have been published up to now. In addition to the data collected at 223 K for the metastable phase of benzophenone, the structure of the stable phase was also determined at 223 K to allow a better comparison of the molecular packing of the stable and metastable modi®cations. Despite considerable experimental endeavour, attempts to grow a single crystal of the metastable phase suitable for structure analysis were initially unsuccessful. Hence, the metastable phase of benzophenone was regarded as an ideal test case with which to evaluate a new method for determining crystal structures from X-ray data derived from micro- crystalline powders. Although a single-crystal structure was subsequently obtained, the two methods of structure deter- mination were carried out quite independently. It is empha-