Molecular dynamics simulations of polyphosphazenes: Poly[bis(chloro)phosphazene][NPCl2](n)

Suitable parameter sets for the CHARMm force field were derived for the structural units in polychlorophosphazene [P=N, P-N, P-CI] using the Dinur-Hagler energy second derivative procedure based on quantum mechanical SCF calculations using the 6-31G* basis set. To validate the reliability of the parameter set, structural results obtained with CHARMm for the adopted model compounds (OP2NCl5 and OP3N2Cl7) were compared with those derived from ab initio quantum mechanics using the 6-31G* basis set. Application of molecular dynamics (MD) simulations in combination with the available X-ray diffraction data provided structural and conformational information on the polymer. The calculation made using the periodic boundary conditions (PBC) agree well with the polychlorophosphazene ordered in a monoclinic unit cell (a=5.98, b=12.99, c=4.92 Angstrom; beta=111.7 degrees). This model was stabilized mainly by the image atoms contribution to the electrostatic energy term and had a quasi-planar conformation of the backbone chain (glide symmetry). The MD calculations also provided evidence that the difference between single and double PN bonds is less marked than that measured experimentally. This result is, however, in agreement with more recent and accurate X-ray studies on poly(methylphosphazene). Validation of the polymer model provided a complete picture, otherwise experimentally inaccessible, of the internal fluctuations of the polymeric chains.