Structure determination of oxamic acid from laboratory powder X-Ray diffraction data and energy minimization by DFT-D

Abstract The structure determination from laboratory X-ray powder diffraction data of oxamic acid, a derivative of the biologically active oxalic acid and the simplest organic acid containing an amide group, is presented in this contribution. An energy minimization analysis by DFT-D was carried out. The structure was determined with the program TALP and refined by the Rietveld method with GSAS-II in space group Cc (No. 9). The final unit-cell parameters are a  = 9.4989 (6), b  = 5.43796 (9), c  = 6.8637 (9) A, β = 107.152 (5)°, V  = 338.772 (10) A 3 , Z  = 4. The refinement converged to the figures of merit: R e  = 0.01674, R p  = 0.03270, R wp  = 0.05652 and GoF  = 3.378 with a good fit between the calculated pattern of the structural model and the experimental pattern. The DFT-D calculations show an excellent reproduction of the experimental structure, validating the correctness of the structure. The non-centrosymmetric nature of the structure was confirmed by SHG measurements. In the crystal structure, the molecules are close to planar and form a complex 2D hydrogen-bonding pattern based on the cyclic amide-acid heterosynthon.