Normal coordinate and vibrational analyses of 2-aza- and 2-phosphabutadienes

The structural stability of 2-azabutadiene CH₂=CHN= CH ₂ and 2-phosphabutadiene CH₂=CH-P=CH₂ were investigated by DFT-B3LYP and ab initio MP2 calculations with 6-311+G** basis set. From the calculations the two compounds were predicted to exist predominantly in a planar trans structure. The calculation at MP2 level of 2-azabutadiene was consistent with the possibility of very low abundance of secondary high energy gauche conformation (taking into consideration the double degeneracy of the non-planar structure). From MP2 caculations the equilibrium constant for the gauche ↔ trans interconversion in 2-azabutadiene was estimated to be 0.111 at 298.15 K, which corresponds to an equilibrium mixture of 10% gauche and 90% trans of the aza compound. The vibrational frequencies were computed at the MP2 level for each of the two molecules in their trans conformation. Complete vibrational assignments were made on the basis of normal coordinate analyses for 2-aza- and 2-phosphabutadienes.