Nonspherical Electron Density Analysis Using X - and Neutron Diffraction Data
Abstract
Electron density distributions in four compounds have been analyzed by the simultaneous modeling of X-ray and neutron diffraction data. The compounds, DHBP (2,4 -dihydroxybenzophenone, C_{13} H_{10}O _3), HAN (hydroxylammonium nitrate, NH _3OH^{+}NO _3^{-}), KPDS (potassium peroxydisulfate, K_2S_2 O_8) and topaz (Al _2(SiO_4)F _2) represent a wide range of atom-atom interactions and provide a stringent test for a single electron density model. In addition, TCNE (tetracyanoethylene, C _6N_4) was reinvestigated at a reduced temperature (-110 ^circC) using X-ray data alone. It was found that the X-ray and neutron models converge to a single model when appropriate considerations have been included and that the X+N technique is helpful in reaching this point. Model considerations which were explored included anharmonic thermal motion (third and fourth order), secondary extinction correction, proper weighting of the two data sets and application of charge constraints. In addition, the multipole deformation functions of the X-ray model were found to be considerably expanded (i.e., more diffuse) for second-row atoms (Al, Si and S), and for first-row atoms in the presence of formal ionic charges. In all cases, the final model (neutron + X-ray multipole) converged with an R-factor of 2.5% or less and left no features in a residual electron density maps. These models represent the basis for a quantitative analysis of the relationships of chemical bonding theories and electron distributions. An ab initio molecular orbital calculation for DHBP is included and the results of this study are compared with experimental model results.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- 1991
- Bibcode:
- 1991PhDT........11X
- Keywords:
-
- X RAY DIFFRACTION;
- ELECTRON DENSITY;
- Physics: Condensed Matter