DFT-B3LYP versus MP2, MP3 and MP4 calculations of the structural stability of azidoketene O = C = CH-NNN

The conformational stability of azidoketene OCCH-NNN was investigated by Density Functional B3LYP and ab initio second-, third- and fourth-order Möller Plesset calculations with the 6-311+G** basis set. Azidoketene was predicted at the DFT-B3LYP level to have the s-cis⇔s-trans conformational equilibrium with the s-trans (O=C=C and NNN moieties are trans to each other) being the predominant low energy form, while at the MP2, MP3 and MP4 levels to have the gauche (O=C=C and NNN moieties are near trans to each other) form as the minimum in the potential curve of the molecule. From full structural optimization of the ground and transition states at the DFT-B3LYP level, the trans-cis barrier was calculated to be about 2 kcal/mol. The vibrational frequencies for the cis and trans conformations were computed for the gauche form at the MP2 level. The calculated infrared and Raman spectra of the molecule were plotted. Complete vibrational assignments were provided on the basis of normal coordinate calculations for the molecule.