CONFORMATIONAL STABILITY AND FORCE-FIELD OF SHORT-CHAIN LINEAR CHLOROPHOSPHAZENES - MNDO CALCULATIONS, P-31 NMR, VIBRATIONAL-SPECTRA, AND NORMAL COORDINATE ANALYSES OF CL3PN(PCL2N)NP(O)CL2 AND [CL3PN(PCL2N)NPCL3][PCL6] (N=1, 2)

The Raman spectra of Cl3PN(PCl2N)nP(O)Cl2 and [Cl3PN(PCl2N)nPCl3]+PCl6-(n = 1, 2) were recorded in the solid and liquid states at different temperatures. The qualitative depolarization ratios were obtained in the liquid phase. A P-31 NMR study for the molecular compounds showed a coalescence phenomenon near 220 K. The potential energy around the PN bonds for the Cl3PN(PCl2N)P(O)Cl2 molecule and [Cl3PN(PCl2N)2PCl3]+ cation are derived from MNDO (modified neglect of diatomic overlap) calculations. The stable conformations are found to be trans-cis for Cl3PN(PCl2N)P(O)Cl2 and [Cl3PN(PCl2N)2PCl3]+. The calculated structural parameters agree well with the X-ray experimental data. The frequencies obtained by normal coordinate analysis are in good agreement with the observed ones. The MNDO calculation of the harmonic force field is in reasonable agreement with the experimental values. The force constant values assigned to torsional modes around the PN bonds correspond to low barriers for the internal rotations. The easy internal rotation around the P-N and P=N bonds can explain the flexibility of the phosphazene backbone a d the elastomeric properties of the phosphazene polymers.