Atomic properties of N₂O₄ based on its experimental charge density

Nitrogen dioxide, being known to exist as a dimer N₂O₄ in the crystal with a very long N-N bond length of 1.76 Å, was crystallized at low-temperature conditions on a diffractometer. High-resolution X-ray data (sin(θ/λ) = 1.249 Å^-1) were recorded with a CCD area detector to allow the generation of an experimental charge density distribution. By making use of Bader's AIM theory, zero-flux surfaces were calculated, and we examined atomic volumes and atomic charges obtained from this experiment and various theoretical calculations. Four commonly used methods of computing atomic charges (Mulliken, AIM, NPA, and CHELP) were considered. The AIM charges are rather independent from the used basis set. Interestingly, the evaluated atomic volumes are very similar between experiment and theory, although in theory isolated molecules are considered. For the long N-N bond a bond order n of approximately 0.5 was derived from a comparison with appropriate model compounds.