Ab initio calculations with full electron correlation by the perturbation method to second order and hybrid density functional theory calculations by the B3LYP method utilizing the 6-31G* and 6-311+G** basis sets have been carried out for the XN 3 molecules (X=H, F, Cl, Br). From these calculations, force constants, infrared intensities, Raman activities, depolarization ratios, vibrational frequencies, and structural parameters have been determined. These calculations support the assignment of the HN 3 and DN 3 out-of-plane bending modes (ν 6, A″) at 602 and 591 cm −1, respectively, which was proposed [J. Chem. Phys. 44 (1966) 4108] earlier and show the recent assignments at 637 and 638 cm −1 [J. Mol. Struct. (Theochem) 434 (1998) 1], respectively, for these modes are in error. Also the usually used frequency of 588 cm −1 for ν 6 for the HN 3 from nitrogen matrix studies for comparison to ab initio predicted values is also clearly in error. The predicted intensity of ν 6 is only 0.2–0.6 km/mol which is 10 3 times less than the stronger bands which explains why it is so weak in the nitrogen matrix compared to its intensity in the gas phase where it drastically increased by Coriolis coupling with ν 5. For ClN 3 there is extensive coupling of the N 3 deformation at 719 cm −1 with the Cl–N stretch at 545 cm −1 where the latter mode is 71% Cl–N stretch. For BrN 3 the coupling is mainly between the N 3 deformation (682 cm −1) and the BrNN bend which is predicted in the 180 cm −1 region. The predicted force constants for FN 3 are compared to those obtained earlier from frequency and distortion data. By combining previously reported rotational constants for HN 3, FN 3 and ClN 3 with the ab initio MP2/6-311+G** predicted parameters, adjusted r 0 parameters have been obtained for all three molecules. The structural parameters for hydrazoic acid are: r(H–N), 1.015(5); r(N 1N 2), 1.243(5); r(N 2N 3), 1.134(2)Å; ∠HN 1N 2, 108.8(5); ∠N 1N 2N 3, 171.8(5)°. It is believed that these reported distances and angles have much lower uncertainties than those previously reported from the microwave data alone.