Topological analysis of experimental electron densities .1. The different C-C bonds in bullvalene

The static electron density based on the rigid-pseudoatom model is extracted from low-temperature, high-resolution X-ray diffraction data of bullvalene (tricyclo[3.3.2.0(2.8)]deca-2,5,8-triene). A comparative analysis of the anisotropic displacement amplitudes based on this and the earlier neutron diffraction study reveals bias in the neutron parameters possibly due to inadequate treatment of extinction. The topological properties of the experimental charge distribution are compared to those obtained from wave functions at different levels of theory. The agreement reached is excellent, especially for the values of the electron density at the bond critical points. The four different e-C bond types can clearly be established. All ring and cage critical points can also be located, confirming the reliability of the experimental density even far away from the nuclei. The model dependence of the topological properties is also discussed, and the importance of the proper treatment of the hydrogen atoms is shown.