Vibrational analyses and C-N rotational barrier in N-vinyl and N-(2,2-dichlorovinyl)nitrones

The conformational and structural stability of N-vinyl nitrone CH₂=CH-N(O)=CH₂ and N-(2,2-dichlorovinyl)nitrone CCl₂=CH-N(O)=CH₂ were investigated by DFF-B3LYP and MP2 calculations with 6-311 + G** basis set. The molecules were predicted to have the planar structure as a result of pronounced conjugation between C=C and N=C bonds. N-vinyl nitrone was predicted to exist predominantly in the trans (C=C and N=C moities are trans to each other) conformation, while the dichloride to exist in a mixture of the gauche and the trans conformations with the former being the lower energy form. The trans to gauche barrier was calculated to be about 6 kcal/mol in N-vinyl nitrone, while the gauche to trans barrier to be about 2 kcal/mol in N-(2,2-dichlorovinyl)nitrone. The vibrational frequencies were computed at the DFT-B3LYP level and the calculated infrared and Raman spectra of the two molecules were plotted. Complete vibrational assignments were made on the basis of normal coordinate analyses for both molecules.