Molecular structures and conformations of polyphosphazenes - A study based on density functional calculations of oligomers

Density functional calculations were carried out on a series of oligomers of polyphosphazenes to characterize molecular structures and conformational properties of the polymers in isolation. Polyphosphazenes are found to have alternating P-N bond lengths in the planar cis-trans backbone conformation. The magnitude of the alternation is in the range of 0.03-0.07 Angstrom, depending on the substituents. The P-N bond alternations are not present in the optimized helical chains. Rotation around the P-N bond is very flexible; the barrier height to rotation of the P-N bond in monophosphazene is about 2 kcal/mol. Systematic study of conformational properties of the oligomers suggests that isolated polyphosphazenes prefer twisted helical structures. The energy differences between the twisted and planar cis-trans chains are ca. 5 kcal/mol per repeat unit. Intramolecular electrostatic interactions are found to be the dominant forces which determine the conformational states. The impact of different side chain substituents (halogen and organo side groups) was also studied. A study of the electronic structures of phosphazene suggests that the P-N bonding consists of an ionic sigma bond and a pi bond mainly induced by negative hyperconjugation: pi(N) -> sigma*(P-X). The new bonding model supercedes earlier ones and explains the structural and conformational properties reported in this and previous work on phosphazenes.