Toward a planar sigma(3)-phosphorus

Correlated ab initio calculations show that the near 20 kcal/mol inversion barrier about phosphorus in phosphole can be reduced to 4.0 kcal/mol (CCSD(T)/6-311G(2D)//MP2/6-311G(2D)) by incorporating two sigma(2),lambda(3)-phosphorus atoms into the five-membered ring, which decrease the ring strain. If the sigma(2),lambda(3)-phosphorus atom is situated in alpha-position from the sigma(3)-phosphorus atom, the planar structure is more stable with respect to the minimum energy structure than with a beta-positioned sigma(2),lambda(3)-phosphorus atom. The planar structures obtained show significant bond length equalization, characteristic for aromatic compounds. For systems having low inversion barrier, the bond length equalization is still significant for the nonplanar structures, which are minima on the potential energy surface. The nonbonding orbital of phosphorus in these cases is mixed considerably with the pi-system. The bond lengths tend toward equalization. These facts indicate that aromatic stabilization has a considerable impact on these nonplanar molecules and is presumably influencing their chemical reactivity as well.