THE CHARGE DENSITY OF DL-ASPARTIC ACID. R. Flaig, T. Koritsánszky, P. Luger, D. Zobel, Institute for Crystallography, Freie Universität Berlin, Germany

A full topological analysis [1] of the electron density of DL-aspartic acid extracted from low temperature (20 K) X-ray data collected with solid state detector and AgK[[alpha]] radiation has been completed and the results compared with those obtained from Hartree-Fock wavefunctions.

The multipole refinement of the diffraction data was performed with the XD program package [2]. The static elecron density, its Laplacian and var-ious one-electron properties were derived. All critical points of the electron density and the Laplacian were located to characterize the covalent bonds and intermolecular interactions.

Experimental topological parameters for C-O bonds were found to be sensitive to the refinement model applied in the multipole treatment of the X-ray data. The ab initio calculations [3] at the Hartee-Fock level show considerable basis set dependence of the bond topological parameters. To mimic the intermolecular forces in the crystal calculations on molecular clusters were also carried out. The effect of the inclusion of the neighbouring molecules on the topology of the charge density especially in the C-O bond has been analyzed.

A comparative study based on diffraction data collected with solid state detector and MoK[[alpha]] radiation is in progress.

[1] R. F. W. Bader, Atoms in Molecules, Clarendon Press, Oxford (1994)

[2] T. Koritsánszky et. al.: XD, A Computer Program Package for Multipole Refinement and Analysis of Charge Densities from X-ray Diffraction Data

[3] Gaussian 92/DFT, Rev. G. 1, M. J. Frisch et. al., Gaussian Inc., Pittsburgh PA, 1993