Polarisation transfer between heteronuclei: application of 31P, 15N polarisation transfer techniques in 15N NMR spectroscopy of phosphorus-nitrogen compounds

Abstract

A systematic study of sensitivity enhancement in 15N NMR spectra of phosphorus-nitrogen compounds by 31P, 15N polarisation transfer was performed using tris-(dimethylamino)phosphine (1) as a model compound. The application of several multipulse techniques (refocussed and non-refocussed INEPT, DEPT) and decoupling schemes shows that maximum enhancement was reached with the non-refocussed INEPT sequence, whereas the performance of longer pulse sequences was deteriorated by relaxation effects. The practical application of the technique was demonstrated for the determination of 15N data of low coordinate phosphorus-nitrogen compounds (iminophosphines and imidometaphosphate ions). In favourable cases, the sensitivity gain may be comparable to that of 1H, 15N INEPT measurements via long-range couplings. In addition to the 1D experiments, the practicability of heteronuclear correlation spectroscopy was demonstrated by the measurement of 2D 31P,15N shift-correlated spectra of 1.