X-Ray Diffraction and Charge Distribution: Application to the electron density distribution in the Hydrogen Bond

Abstract

The electron density distribution (EDD) in molecules closely resembles the joint EDD*s of the atoms that constitute the molecule. In fact, the deviations are so small that for more than fifty years the scattering by X-rays could be accounted for by using atomic scattering factors. Better recording techniques and improved methods for analysis of the measured intensities allow a more accurate determination of the EDD. The last twenty years has seen many studies of the effect of covalent bonding on the EDD of molecules in crystals. The results show in general good qualitative agreement with the outcome of Hartree-Fock calculations. Although the number of quantitative studies, in which ab-initio structure factors are compared with experimental ones, is still small, we assume that a careful diffraction experiment will yield quantitative information on the charge distribution in molecules. The reliability of the results strongly depend on the way the experiment is carried out and the type of information that is wanted. To illustrate the possibilities to obtain useful information by diffraction experiments we study a number of simple systems: atoms and molecules.