Normal coordinate analyses and barrier to internal rotation of nitroso- and nitroazides

The conformational and structural stability of nitrosoazide NNN-N=O and nitroazide NNN-NO2 were investigated by DFT-B3LYP and ab initio MP2 calculations with 6-311++G** basis set. From the calculations, nitrosoazide was predicted to exist predominantly in the planar trans (NNN and N=O groups are trans to each other) structure with high trans-cis rotational barrier of about 11 kcal mol-1 as a result of pronounced conjugation between the azide group and the N=O bond. The NO2 rotational barrier in nitroazide was predicted from the symmetric potential function to be of about 7 kcal mol-1. The vibrational frequencies were calculated at the DFT-B3LYP level and the infrared and Raman spectra of the cis-trans mixture were plotted. Complete vibrational assignments were made on the basis of normal coordinate calculations for the stable conformers of both molecules. For nitrosoazide, the calculated wavenumbers were compared to the corresponding experimental values obtained from early reported Raman spectrum of the molecule.