DERIVATION OF FORCE-FIELD PARAMETERS FOR BDA(5),6-LAMBDA(5)-CYCLOTRIPHOSPHAZA-1,3,5-TRIENES AND BDA(5),6-LAMBDA(5)-CYCLOTRIPHOSPHAZA-1,3,5-TRIENES

Force field parameters for use in MM2 and CHARMm programs for the titl e cyclic phosphazenes have been developed. The Dinur-Hagler energy sec ond derivative approach, based on SCF calculations at the 4-31G level on a model compound, has been applied to achieve the related paramete rs by a fitting procedure. A useful method for obtaining energy second derivatives, with respect to only one valence coordinate from the car tesian hessian matrix is also presented. The MM2 and CHARMm structures of the cyclic model compound calculated by the new force field parame ter set agreed with the structure deduced from ab initio calculation. Some differences between the empirical force field and quantum mechani cal methods were found on torsion angles defined by at least three ato ms of the ring. This could be attributed to the non-cyclic structure o f the model compound required to extract the information for torsion a ngle interactions. Calculated and X-ray structures of 2,2,4,4,6,6-hexa kis-(phenoxy)-2 lambda(5),4 lambda(5),6 lambda(5)-cyclotriphosphaza-1, 2,5-triene were in reasonable agreement. The conformations about P-O a nd O-C bonds in the solid were reproduced less well owing to the expec ted larger effect of the crystal environment on torsional angles corre sponding to the exocyclic molecular fragments.