FLIPPEN-ANDERSON, GEORGE AND GILARDI 1125 Related literature. A search of the most recent Cam- bridge Structural Database file did not produce any similar structures. This work was supported in part by contract # N00173-86-M-R828 from the ONR Mechanics Division. References International Tables for X-ray Crystallography (1974). Vol. IV. Birmingham: Kynoch Press. (Present distributor Kluwer Academic Publishers, Dordrecht.) SrmLDRICK, G. M. (1980). SHELXTL80. An Integrated System for Solving, Refining, and Displaying Crystal Structures from Diffraction Data. Univ. of Grttingen, Federal Republic of Germany. Acta Cryst. (1990). C46, 1125-1127 Structure of (S,S)-3,6-Diphenyl-l,4-dioxane-2,5-dione BY V. M. LYNCH, J. POJMAN, J. K. WHITESELL AND B. E. DAVIS Department of Chemistry, University of Texas at Austin, Austin, TX 78712, USA (Received 24 October 1989; accepted 4 January 1990) Abstract. CI6H1204, Mr=268"27, orthorhombic, P2~2~2~, a = 5" 103 (3), b = 9.272 (6), c = 26.50 (3) A, V = 1254 (2) A 3, Z = 4, Dx = 1.42 g cm- 3, a(Mo Ka) =0.7107A, /z=0-9574cm -~, F(000)=560, T% 198 K, R = 0.0542 for 886 reflections [Fo >-4tr(Fo)]. The absolute configuration is assigned on the basis of internal comparison to the (+)-mandelate moiety. Close C--H...O contacts are observed resulting in a two-dimensional network of H-bonded molecules lying parallel to ab. Atom H3 is involved in a three-center C--H...O-type hydrogen bond. The shorter H...O contact involves C3--H3.-.O8 (1 -x, ½+y, 3 z) with H3--.O8 2.26(6)A, C3...O8 2-949 (7)A and C--H...O 127 (4) °, while the longer contact involves C3--H3...O7 (x-1, y, Z) with H3...O7 2.41 (6) A, C3--.O7 3.143 (8) A and C--H...O 132 (4) °. Experimental. (1) was prepared by p-toluenesulfonic acid catalyzed self-condensation of S-(+)-mandelic acid in benzene at reflux with azeotropic removal of water (Pojman, 1988). After recrystallization from tetrahydrofuran, the material was sublimed in vacuo providing single crystals suitable for X-ray analysis. The data crystal was a colorless needle of approxi- mate dimensions 0-03 x 0-13 x 0.54 mm. The data were collected on a Nicolet R3 diffractometer using a graphite monochromator and equipped with a Oh O (1) 0108-2701/90/061125-03503.00 H Nicolet LT-2 low-temperature assembly. Lattice parameters were obtained from the least-squares refinement of 36 reflections with 12.7 < 20 < 21.3 °. The data were collected using the to-scan technique (2444 reflections, of which 1323 were unique, Rin t = 0.0412 from averaging hkl and hl~i reflections), with a 20 range from 4.0-50.0 °, and a 1.0 ° to scan at a scan rate of 1-4 ° min -~ (h=0---,6, k = -11--,11, l = -31----31). Four reflections (006; 110; 123; 021) were remeasured every 96 reflections to monitor instru- ment and crystal stability (maximum correction on I was < 4%). The data were also corrected for Lp effects but not for absorption. The decay correction and the data reduction were performed using SHELXTL-PLUS (Sheldrick, 1987). Reflections having 17o< 4tr(Fo) were considered unobserved (437 reflections). The structure was solved by direct methods (Sheldrick, 1987) and refined by full-matrix least squares (Sheldrick, 1976). The non-H atoms were refined with anisotropic thermal parameters. All H-atom positions were obtained from a AF map and refined with isotropic thermal parameters. After several cycles of refinement, the Uiso for H3 refined to a negative value and, therefore, was fixed at 1.2 x Ue~ of C3 for the final refinement cycles. In all, 228 parameters were refined. The function Y.w(IFol- IF~I) 2 was minimized, where w = 1/[tr(Fo)] 2 and tr(Fo) =0.5kI-~/2{[o-(I)]2+ (0-021)2} 1/2. The intensity I is given by (/peak- /background)X (scan rate); 0-02 is a factor to downweight intense reflections and to account for instrument instability and k is the correc- tion due to Lp effects and decay, tr(/) was estimated from counting statistics; or(/) = [(/peak + /background) 1/2 x (scan rate)]. The final R = 0-0542 for 886 reflec- tions, with wR=0-0487 (Ra,=0"0919, wRan = 0"0557) and goodness of fit = 1-250. The maximum IA/trl < 0"1 in the final refinement cycle and the © 1990 International Union of Crystallography