Valence Structure from Coherent X-Ray Scattering: Fourier Difference Synthesis

The one-electron density function for a molecule of first-row atoms is partitioned into core-electron and valence-electron density pieces. The SCF 1s AO's are used to define the core-electron density pieces. The residual density, after removal of the core density, constitutes the valence density, which is assumed to be chemically interpretable. The partitioning is explored in reciprocal space to provide insight into x-ray diffraction experiments. The contribution from valence scattering within the CuKᾱ sphere is 10%–30% of the total scattering. Valence-electron density maps have been Fourier synthesized from x-ray diffraction data of uracil. The density maps reveal trigonal bonding for the nitrogen atoms and bridge densities in the middle of the C(5)–C(6) and C(4)–C(5) bonds. The densities near the time-average nuclear positions are unreliable.