Pyramidality and aromaticity in polyphosphaphospholes

Ab initio calculations show that, for the P_n(CH)_4-nPH with n = 0 - 4, the fully optimized structures have decreasing pyramidality at the tri-coordinated phosphorus atom with n. The interaction between the tri-coordinated phosphorus and the carbons-phosphorus π-system with different n, the differences of the structure, electron properties and energy among the phospholes indicate that the ring strain and the delocalization of the lone pair electron of tri-coordinated phosphorus should be responsible for this phenomena. Various aromatic criteria such as geometry, magnetism, and energy show that these molecules are aromatic, and their aromaticities increase with the number of phosphorus atoms in the nonplanar pentagons. The electron circulation in the aromatic molecules can promote the delocalization of the lone pair. In contrast to the B3LYP prediction, we draw some similar conclusions, but the planarity and aromaticity of molecules increase largely at the MP2 lever due to the consideration of electron correlation.