1308 C4H2F2N203 AND CloHvFN203 C(2)--N(1)---C(6) 128.0 (4) C(2)--N(3)---C(4) 125.7 (4) O(2)----C(2)--N(1) 122.3 (4) O(2)---C(2)--N(3) 122.5 (4) N(I)----C(2)---N(3) 115.2 (4) O(4)---C(4)--N(3) 122.3 (4) O(4)----C(4)--C(5) 120.6 (4) N(3)---C(4)--C(5) 117.0 (3) F(1)---C(5)----C(4) 106.8 (3) F(1)--C(5)--C(6) 106.4 (3) C(4)---C(5)---C(6) 113.3 (3) F(1)----C(5)---C(7) 108.7 (3) C(4)----C(5)--C(7) 111.9 (3) C(6)---C(5)----C(7) 109.5 (3) O(6)---C(6)---N(1) 122.8 (4) O(6)---C(6)---C(5) 121.7 (4) N(1)---C(6)---C(5) 115.5 (3) D--H...A D. • .H D. • .A D--H. • .A o(4i)---H(1).- -N(I~) 2.06 (5) 2.867 (5) 174 (1) O(6~i)--H(3) .. .N(3 u) 2.00 (5) 2.899 (5) 162 (1) Symmetry codes: (i) x + ½, ½ - y, -z; (ii) x - ½, ½ - y, -z. The structures were solved by direct methods and refined by full-matrix least squares. The non-H atoms were refined anisotropically; H atoms were located by standard techniques and were refined isotropically [for (2)], or were placed in optimized positions (C--H = 0.96~) as a riding model contribution to the structure-factor calculations [for (1); group isotropic displacement parameter = 0.08 (2)A2]. Corrections were applied for Lorentz and polarization effects, and real and imaginary anomalous dispersion (Cromer, 1974). Structure solution, refinement and the calculation of derived results were performed using SHELXTL (Sheldrick, 1986). Lists of structure factors and anisotropic displacement parameters for (1) and (2), complete geometry for (2) and packing diagrams for (1) and (2) have been deposited with the British Library Document Supply Centre as Supplementary Publication No. SUP 71818 (16 pp.). Copies may be obtained through The Technical Editor, International Union of Crystallography, 5 Abbey Square, Chester CH1 2HU, England. [CIF reference: ST1077] Acta Cryst. (1994). C50, 1308-1312 Substituted Diphenyl Oxalates CLIFFORD GEORGE* Laboratory for the Structure of Matter, Naval Research Laboratory, Washington DC 20375, USA P a u l M. LAHTI,* DAVID A. MODARELLI AND AHMET INCELI Department of Chemistry, University of Massachusetts, Amherst, MA 01003, USA (Received 19 April 1993; accepted 2 November 1993) Abstract The ethanedioic acid (oxalic acid) diphenyl ester derivative with bis(2,6-tert-butyl) substitution, bis- (2,6-di-tert-butyl-4-methoxyphenyl) ethanedioate, C32H460 6 (3), adopts an s-trans oxalate conforma- tion due to steric constraints; such derivatives consti- tute conformationally well defined molecular structural units. Diphenyl ethanedioate, C14H1oO4 (1), has approximate s-cis geometry and diphenyl ethanedioate with tert-butylation on only one ring, 1-(2,4,6-tris-tert-butylphenyl) 2-phenyl ethanedioate, C26H3404 (2), has a gauche conformation. References Bolton, W. (1963). Acta Cryst. 16, 166-173. Cromer, D. T. (1974). International Tables for X-ray Crystallogra- phy, Vol. IV, Table 2.3.1. Birmingham: Kynoch Press. (Present distributor Kluwer Academic Publishers, Dordrecht.) Cromer, D. T. & Waber, J. T. (1974). International Tables for X-ray Crystallography, Vol. IV, Table 2.2B. Birmingham: Kynoch Press. (Present distributor Kluwer Academic Publishers, Dordrecht.) DesMarteau, D. D., Resnati, G., Favretto, D. & Traldi, P. (1992). Org. Mass Spectrom. 27, 204-210. Harvey, S. C. (1985). Goodman and Gilman's The Pharmacological Basis of Therapeutics, 7th edition, edited by A. G. Gilman, L. S. Goodman, T. W. Rail & F. Murad, ch. 17, pp. 339-371. New York: McMillan. Larson, A. C. (1970). Crystallographic Computing, edited by F. R. Ahmed. Munksgaard: Copenhagen. Meester, P. de, Jovanovic, M. V., Chu, S. S. C. & Biehl, E. R. (1986). J. Heterocycl. Chem. 23, 337-341. Resnati, G. (1990a). Farmaco, 45, 1043-1044. Resnati, G. (1990b). Farmaco, 45, 1137-1139. Resnati, G. & DesMarteau, D. D. (1992). J. Org. Chem. 57, 4281-4284. Sheldrick, G. M. (1986). SHELXTL User's Manual. Nicolet XRD Corporation, Madison, Wisconsin, USA. Welch, J. T. (1987). Tetrahedron, 43, 3123-3197. © 1994 International Union of Crystallography Printed in Great Britain - all rights reserved Comment During studies of radical pairs photochemically generated in neat solid diphenyl ethanedioates (more commonly called oxalates, a usage we retain in this work), some of us (Modarelli, George & Lahti, 1991) wished to study the crystallography of these com- pounds as a function of different substitution pat- terns. We found that triplet radical-pair electron- spirt-resonance (ESR) spectra could be most readily detected in diphenyl oxalates with 2,6-di-tert- butylated phenyl rings. More extensive ESR studies of such radical pairs in oriented crystalline arrays required definitive structural information. In this report, we describe the crystal structures and varia- tion in solid-state molecular conformation of the following substituted oxalates: diphenyl oxalate (1), phenyl 2,4,6-tris-tert-butylphenyl oxalate (2) and bis(2,6-di-tert-butyl-4-methoxyphenyl) oxalate (3). R) R~ R.~ R~ R2 R" (i) R,, = R,~ = H (2) R,, = 'Bu; R,; = H (3) RI = R2 = R~ = R2 = 'Bu; R3 = R.( = MeO Acta Co,stallographica Section C ISSN 0108-2701 ©1994