TOPOLOGICAL ANALYSIS OF THE EXPERIMENTAL ELECTRON-DENSITIES OF AMINO-ACIDS - 1 - D,L-ASPARTIC ACID AT 20 K

High-resolution X-ray diffraction data collected at 20 K are interpret ed in terms of the rigid-pseudoatom formalism to derive the electron d ensity and related properties, such as the electrostatic potential and electric moments, of the crystalline D,L-aspartic acid. The refinemen t models applied are restricted via rigid-bond type constraints to red uce possible bias in the mean-square displacement amplitudes due to in adequacies in the thermal deconvolution. The density and its Laplacian extracted from the data is analyzed in terms of the topological prope rties of covalent bonds and nonbonded interactions. The results are co mpared to those calculated at the Hartree-Fock level of theory and to those obtained experimentally for analogous molecules. The comparison must consider the differences in the locations of the bond critical po ints of the densities in question, that is, how the bond polarity mani fests itself in the distribution of charge obtained by different metho ds. One of the key questions to the reliability of experimental pseudo atomic densities seems to be whether the treatment of the X-ray data c an be standardized so as to reduce model inadequacies, especially thos e related to the derivation of monopole populations.