The crystal structure of mesaconic acid, C5H604, has been determined from 491 X-ray intensities, and refined to R = 9.5 %. The crystals are monoclinic with a= 7"225, b = 11.88, c = 7.00 A, ,8= 98°40 ', space group P21/c, Z= 4. The structure consists of infinite chains parallel to a, adjacent molecules being linked by hydrogen bonds of length 2.63 and 2.74 A,. The longer of thcse is a bifurcated hydrogen bond, with its partner of length 2-88 ~ holding the chains together. The C=O bonds in the molecule are in the cis configuration.As part of a programme of studying hydrogen-bonded systems, we have investigated mesaconic acid, also called methylfumaric acid. The present work describes the crystal and molecular structure and adduces evidence to show that the name methylfumaric acid is to some extent misleading, since the C=O bonds are not in the trans configuration.
ExperimentalThe crystals were grown from ethanol. They were platy in habit with face (010), and show extinction at an angle of about 16 ° with the long edge [001]. Cell dimensions were obtained from high sin 0 reflexions on Weissenberg films calibrated with silver lincs. The crystal data are a=7"225, b= 11-88, c=7.00 ~, fl=98°40 ', do= 1.44, dc=1.45 g.cm -3, Z=4, space group P21/c, p(Cu K~)= 11"5 cm-491 independent reflexions were collected by Weissenberg photography. The intensities were measured visually and brought to an approximate absolute scale by statistical methods.
Determination and refinement of the structureThe structure was solved by interpretation of Patterson projections. The c projection had a marked similarity to one projection of fumaric acid on which one of us (M.P.G.) had worked earlier, and the crystal structure of which is now well established (Brown, 1966; Bednowitz & Post, 1966). Assuming similar linkages of molecules in chains, packing considerations gave reasonable trial coordinates. Full-matrix least-squares refinement (Busing, Martin & Levy, 1962) with unit weights for the observed reflexions and anisotropic thermal factors for carbon and oxygen reduced R to 12.7 %. A three-dimensional difference synthesis served to locate the hydrogen atoms, and a structure factor calculation gave a final R of 9.5 %. The positional and thermal parameters are given in Table l(a) and (b). Observed and calculated structure factors are in Table 2. Bond lengths, bond angles, and intermolecular con-