An accurate determination of the structure of ammonium oxamate

The crystal structure of ammonium oxamate (CONH2.COONH4) has been studied using CuKa X-radiation, by means of a three-circle diffractometer incorporating a xenon-filled proportional counter. Accurate three-dimensional intensity data were collected and a least-squares refinement was carried out. The positions of the hydrogen atoms were obtained and refined. A peak of electron density, about half as high as a hydrogen atom, was observed at the centre of the C—C bond and a correction applied for it increased the length of the bond by 0.003 Å. The bond lengths were corrected for librational motion, and the values obtained are C—C =1.564 ±0.002 Å; C—N = 1.324± 0.002 Å; C—O (amidic) = 1.248± 0.002 A; C— O (carboxylate) = 1.257 + 0.003 Å and 1.256 ± 0.003 Å. The oxamate ion is found to be planar, and the ammonium ion tetrahedral. The length of the C—C bond is greater than any theoretical value yet suggested for the length of a single bond between trigonally hybridized carbons atoms.