A study of the electron-density distribution in sodium azide, NaN₃

The crystal structure of sodium azide, NAN₃, has been reinvestigated by X-ray diffraction methods. The electron density of the azide ion as determined using a high-order X-ray refinement is compared with the theoretical density determined in an ab initio molecular-orbital calculation. The wavefunction of the azide ion has been used to calculate improved atomic scattering factors which result in a significant improvement in the refinement. Molecular scattering factors calculated from the wavefunction have been used in rigid-body refinements including librational thermal motion. The molecular refinement results in a significantly better fit than do the atomic refinements. When standard ellipsoidal temperature factors are used false detail is indicated in deformation-density maps. This problem is eliminated when a third-cumulant expansion is used.