The molecular properties of the halogen pseudohalides studied by both ab initio and DFT methods

The dipole, quadrupole, and other second moments have been determined at the equilibrium structures of the halogen azides, isocyanates, and isothiocyanates, for both principal axes and inertial axes. The theoretical procedures used are Möller–Plesset (MP2) and DFT (B3LYP) methodologies, with TZVP and cc-pVTZ basis sets. There is systematic variation in the calculated directions of the dipole moments in the present series, where B3LYP and MP2 methodologies show differences up to ∼20° for the directions. This discrepancy is largest in ClN3, but quite significant for several other compounds, such as XNCO (where X = Cl, Br and I). The dipole moments of the compounds rotate through a wide angle, as the halogen changes; in contrast, the axes of the second moments rotate to a much smaller degree. The properties are compared with the limited microwave spectral data so far available, in the hope that the present study will encourage further experimental study. There is an urgent need for new experimental data on the dipole moment a,b-components for these compounds.