Natural bond orbital (NBO) population analysis of cyclic thionylphosphazenes, [NSOX (NPCl 2) 2]; X = F ( 1), X = Cl ( 2)

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Theoretical study of cyclic thionyl phosphazenes, [NSOX(NPCl 2) 2]; X = F ( 1), X = Cl ( 2) have been performed using quantum computational ab initio RHF and density functional B3LYP and B3PW91 methods (DFT) with the 6-31G(d,p) basis set implemented in the Gaussian-98 program suite. Geometries obtained from DFT calculations were used to perform NBO analysis by the NBO 3.1 program as implemented in Gaussian-98. The results obtained through these theoretical calculations revealed that decreasing occupancy of the localized σ SX orbital in the idealized Lewis structure, or increasing occupancy of σ SX ∗ of the non-Lewis orbital, and their subsequent impact on molecular stability and geometry (bond lengths) are related with the resulting p character of the corresponding sulfur natural hybrid orbital (NHO) of σ SX bond orbital. The natural population analysis (NPA) results reveal strong partial charges separation on the skeletal atoms of molecules 1 and 2 which can be contributed in the stabilization energies of the molecules using the most likely phosphazene bonding alternatives, ionic bonding and its complementary negative hyperconjugation due to interactions of filled NBOs with antibonding NBOs, determined by standard perturbative approach referred as E(2) energies. This approach approves anticipating that the electrostatic repulsion or attraction between atoms gives a significant contribution to the intra-and inter-molecular interactions.