Refinement of DL-aspartic acid

Abstract

Crystals of DL-aspartic acid (C4NO4H7) were grown at room temperature from an aqueous solution of the commercially available sample. The crystals be- long to the monoclinic system with cell dimensions a= 18.947+0.002, b= 7.433 + 0.001, c=9.184 + 0.001 A and fl=123.75+0-02 °. The systematic absences are: hkl, h + k = 2n + 1 and hOl, 1= 2n + 1. The space group is C2/c with eight molecules in the unit cell. The cal- culated density is 1.563 g cm -3 and the density ob- served by the method of flotation in a bromoform- benzene mixture is 1.561 g cm -3. Introduction. The crystal structure of DL-aspartic acid (DLASP) has already been determined using the sym- bolic addition procedure for centrosymmetric space groups (Rao, Srinivasan & Valambal, 1968) using visually estimated photographic data. The intensities of reflections out to 20--128 ° were measured on a Picker FACS-I diffractometer using Ni filtered Cu Kc~ radiation in the 0-20 scan mode with a scan rate of 2 ° per min for the 20. The crystal size was 0.2 x 0.2 x 0.3 mm. Both hkl and hf¢l reflections were measured. The data were corrected for Lorentz and polarization fac- tors but not for absorption. The equivalent reflections were averaged and a set of 891 independent reflections was obtained. The R value (= ~IF1-F2I/O'5y_(F I+Fz)) between the two sets of amplitudes was 0.018. The refinement was carried out by minimizing ~w(IFol- IFcl) 2 with the full-matrix least-squares pro- gram of Busing, Martin & Levy (1962), starting with the positional parameters of the heavy atoms deter- mined in the earlier work. The positions of all the hydrogen atoms were determined from a difference electron density map. The heavy atoms were refined with anisotropic thermal parameters and the hydro- gen atoms with isotropic thermal parameters. The final R value for the 851 observed reflections (I> 1.5a) used in the refinement was 0-031. The weighting scheme