Structural stability and vibrational analysis of aminoethylene CH ₂=CH-NH₂ and aminoketene O=C=CH-NH₂

The structural stability of vinylamine (aminoethylene) CH ₂CH-NH₂ and keteneamine (aminoketene) OCCH-NH₂ were investigated by DFT-B3LYP and ab initio MP2 calculations with 6-311+G** basis set. From the calculations vinylamine was predicted to exist predominantly in a non-planar structure consistent with an earlier microwave results, while keteneamine in a mixture of anti and syn conformations, with the anti being the low energy conformer at ambient temperature. The equilibrium constant for the anti⇔syn interconversion in keteneamine was estimated to be 0.4456 at 300 K. From the calculations the NH₂ inversion barrier was estimated to be about 0.5 and 1.6 kcal/mol for vinylamine and about 2.3 and 3.3 for keteneamine at DFT-B3LYP/6-311+G** and MP2/6-311+G** levels, respectively as compared to about 1.0 kcal/mol (356 cm^-1) obtained from a microwave data of vinylamine. The vibrational frequencies were computed at the DFT-B3LYP level and the infrared and Raman spectra for each molecule and its deuterated species. Complete vibrational assignments were made on the basis of normal coordinate analyses and isotopic substitution for vinylamine and keteneamine.