Crystal structure and molecular mechanics, dynamics and quantum-mechanical ab initio studies of 2,2,4,4,6,6-hexakis(p-phenoxyphenoxy)-2λ5,4λ5,6λ5-cyclotriphosphaza-1,3,5-triene

Abstract

Molecular mechanics modelling and molecular dynamic (MD) simulations have been carried out for [NP(OC₆H₄OPh-p)₂]₃. Force-field parameters developed for cyclotriphosphazenes gave calculated bond lengths and angles in agreement with the values determined by X-ray diffraction analysis of the crystal. Calculated and X-ray diffraction data for the torsional angles were in poor agreement due to crystal-packing forces determining the conformation in the solid. The MD simulation gave a nearly symmetric equilibrium conformation and showed that the fluxional mobility of the phenoxy groups increases on going from the P–O bonds of the phosphazene core to the external fragments of the molecule. The crystals were triclinic, space group P, with a= 15.345(2), b= 15.440(3), c= 15.999(3)Å, α= 80.43(1), β= 66.67(1), γ= 60.62(1)° and Z= 2, and the structure was refined to R= 0.057 and R′= 0.066. The force-field parameter set for MD well reproduces the energy difference between the solid and gas-phase conformations calculated by the ab-initio(STO-3G*) method. The calculated small energy differences between the molecular orbitals indicate a possible quasi-band structure for the bonding electrons. A perturbation of the electronic system localized on the atoms of the external phenyl groups can therefore propagate towards the P and N atoms of the cyclophosphazene ring.