Derivation of force field parameters for 2,2,4,4,6,6-hexakis-(alkyloxy)- and -(aryloxy)-2λ5,4λ5,6λ5-cyclotriphosphaza-1,3,5-trienes

Force field parameters for use in MM2 and CHARMm programs for the title cyclic phosphazenes have been developed. The Dinur-Hagler energy second derivative approach, based on SCF calculations at the 4-31G* level on a model compound, has been applied to achieve the related parameters by a fitting procedure. A useful method for obtaining energy second derivatives, with respect to only one valence coordinate from the cartesian hessian matrix is also presented. The MM2 and CHARMm structures of the cyclic model compound calculated by the new force field parameter set agreed with the structure deduced from ab initio calculation. Some differences between the empirical force field and quantum mechanical methods were found on torsion angles defined by at least three atoms of the ring. This could be attributed to the non-cyclic structure of the model compound required to extract the information for torsion angle interactions. Calculated and X-ray structures of 2,2,4,4,6,6-hexakis-(phenoxy)-2λ5,4λ 5,6λ5-cyclotriphosphaza-1,2,5-triene were in reasonable agreement. The conformations about P-O and O-C bonds in the solid were reproduced less well owing to the expected larger effect of the crystal environment on torsional angles corresponding to the exocyclic molecular fragments. © 1995.