Structure of N,N,N',N'-tetrakis(2-fluoro-2,2-dinitroethyl)oxamide by the consistent electron density approach.

C10H8F4N10O18, Mr = 632.22, monoclinic, Pc, a = 7.8885 (6), b = 6.7787 (4), c = 21.595 (2) A, beta = 108.21 (1) degree, V = 1096 (1) A3, Z = 2, Dx = 1.914 g cm-3, lambda(Cu K alpha) = 1.5418 A, mu = 18.7 cm-1, F(000) = 636, T = 293 K, 1791 unique data, 1733 with I greater than 3 sigma (I), R = 0.047. The structure was solved by the consistent electron density approach (CEDA), in which a small starting set of random phases was refined and expanded by the application of restraints to the electron density. A refinable preliminary structure was obtained by fitting a model to a 3 A map, calculated with 27 reflections (all data with d greater than 3 A, and magnitude of Fo greater than 170) whose phases had been determined by the CEDA. The molecule has an extended, open conformation; the two pairs of fluorodinitroethyl substituents located across the molecular center from each other show i and C2 pseudo-symmetry respectively.