15N NMR Spectroscopy of Cyclotriphosphazenes and Polyphosphazenes Based on31P,15N HMQC Correlations

A³¹P and¹⁵N NMR study of cyclotriphosphazenes 1–4 and polyphosphazene 5 has been carried out including solvent and substituent effects.¹⁵N chemical shifts and³¹P,¹⁵N coupling constants are readily obtained through 1D and 2D³¹P,¹⁵N HMQC correlation spectroscopy on natural-abundance samples. This methodology combined with spectral simulation also yields¹⁵N-induced isotope shifts.³¹P and¹⁵N are both relatively insensitive to changes in solvent polarity. Chemical shifts are reasonably explained in terms of electronic density variations, and³¹P,¹⁵N couplings are rationalized by considering their absolute sign, which depends on the substituent bonded to phosphorus.¹⁵N proved to be more sensitive to structural modifications than³¹P. Therefore, the availability of¹⁵N NMR data on cyclotriphosphazenes can be considered a valuable tool in the design of model compounds for polyphosphazene synthesis.