MP2, MP3 and MP4 versus DFT-B3LYP energies and vibrational assignments of near-planar carbamoyl azide and ketene

The structures of carbamoyl azide and ketene were investigated by the Density Functional B3LYP level of theory and ab initio second-, third- and fourth-order Møller–Plesset MP2, MP3 and MP4 calculations with the 6-311G∗∗ and/or the 6-311+G∗∗ basis sets. The amino NH2 group of both molecules was predicted at the DFT-B3LYP level to adopt the planar structure, while at the Møller–Plesset levels of theory to have the non-planar conformation. The vibrational frequencies of the two molecules in their non-planar ground state conformation were computed at the MP2/6-311G∗∗ level of theory. Normal coordinate calculations were then carried out and the potential energy distributions (PEDs) among the symmetry coordinates of the normal vibrational modes of the molecule were calculated. Complete vibrational assignments were provided on the basis of normal coordinate calculations and comparison with reported experimental data of the azide. Better assignment for the low frequency modes was provided by the scaled MP2 wavenumbers than the calculated ones by the B3LYP level of calculation for carbamoyl azide.