REFINEMENT OF THE CRYSTAL-STRUCTURE OF MALONIC-ACID, C3H4O4

A reinvestigation of the X-ray crystal structure of malonic acid (C3H4O4) has been made in the light of some observations arising from solid-state C-13 NMR studies on this compound. The unit cell parameters are: a = 5.156(1), b = 5.341(1), c = 8.407(1) angstrom, alpha = 71.48-degrees (2), beta = 76.12-degrees (2), gamma = 85.09-degrees (2), space group P1BAR, Z = 2. The structure was solved by direct methods and refined by full-matrix least-squares refinement to a final R value of 0.038 for 711 unique reflections with I > 3sigma. The packing of the molecules involves dimeric hydrogen bonded association of each carboxyl group with a centrosymmetrically related neighbor. The angles O-C-O (123.3-degrees and 124.8-degrees, respectively) for the two carboxyl groups agree qualitatively with the values predicted from a linear relationship relating the observed sigma22 component of the Carbon-13 NMR chemical shielding tensor for the carboxyl carbon atom to the bond angle O-C-O at the carbon atom.