CONFORMATIONAL STABILITY AND FORCE-FIELD OF CHLOROMONOPHOSPHAZENES - MNDO CALCULATIONS, VIBRATIONAL-SPECTRA AND NORMAL COORDINATE ANALYSES OF CL3PNP(X)CL2 (X = O, S) AND [CL3PNPCL3][Y] (Y = CL, PCL6)

The potential energy around the PN bonds for the Cl3PNP(X)Cl2 (X = O, S) molecules and [Cl3PNPCl3]+ cation have been derived from MNDO (modified neglect of diatomic overlap) calculations. The most stable conformations are two s-trans isomers in nearly eclipsed forms. The calculated structural parameters agree well with the X-ray experimental data. Barriers of 6 and 1.5 kJ/mol for the rotations of the POCl2 and PCl3 groups are predicted. In addition, the Raman spectra and the qualitative depolarization measurements for these molecules in the liquid phase have been obtained. All the data indicate that the molecules exist as a mixture of two rotamers in the molten phase. These two conformers are stabilized in the crystal packing of Cl3PNP(O)Cl2. The observed frequencies are in good agreement with the calculated values obtained by normal coordinate analysis. The MNDO calculation of the harmonic force field is in reasonable agreement with the experimental values. The force-constant values assigned to the torsional modes around the PN bonds correspond to low barriers for the internal rotations. These easy internal rotations around the P-N and P = N bonds can explain the flexibility of the phosphazene backbone and the elastomeric properties of the polyphosphazene polymers.