CONFORMATIONAL STABILITY AND FORCE-FIELD OF POLYPHOSPHAZENES - MNDO CALCULATIONS, VIBRATIONAL-SPECTRA AND NORMAL-COORDINATE ANALYSES OF [NP(R)2]N (R = CL, OCH2CF3, OC6H5)

Raman and Fourier-transform Raman spectra (1 500 - 100 cm-1) of (PCl2N )n (PDCP) were recorded in the solid phase and in solution at room tem peratuure. Raman (3 500 - 100 cm-1) and infrared (4000 - 200 cm-1) spe ctra of [P(OCH2CF3)2N]n (PDFP) and [P(OC6H5)2N]n (PDPP) were recorded in the solid phase and at different temperatures (in the case of Raman spectroscopy). The conformation of the isolated macromolecule PDCP is assumed to be analogous to the geometry of the Cl2(O)PN(PCl2N)6PCl3 o ligomer optimized by the use of MNDO (modified neglect of diatomic ove rlap) calculations. The optimized cis-trans conformation is in good ag reement with the X-ray experimental data concerning the polymer. The c alculated low energy barriers around the PN bond along the chain axis can explain the flexibility of the phosphazene backbone and the elasto meric properties of the polymers. The MNDO calculation of the harmonic force field of Cl2(O)PN(PCl2N)6PCl3 is in reasonable agreement with t he experimental values for (PCl2N)n in solution as well as in the amor phous phase. The normal coordinate analyses of (PCl2N)n were undertake n according to several structural hypotheses using a force field deriv ed from linear short-chain molecules. The Raman spectra of PDCP in sol ution or in the amorphous phase are in reasonable agreement with the v ibrational frequencies calculated for a planar cis-trans macromolecule and have a striking resemblance with those of linear short-chain anal ogs Cl2(O)PN(PCl2N)nPCl3 (n = 1, 2). The Raman and infrared spectra of the substituted polymers PDFP and PDPP are dominated by the character istic features of the chain substituents.