Natural abundance 15N NMR studies have been carried out on a series of cyclic phosphoramidates, including several methylated three-membered ring versions. The three-membered ring aziridine phosphoramidate was observed to have a substantial shielding (34 ppm) with respect to larger ring systems. Large β-substituent effects on the 15N shift were noted along with changes in directly bonded 15N-31P couplings from 9.3 Hz in the nonsubstituted three-membered ring compound to ≤1 Hz in the fully substituted 2,2,3,3-tetramethylphosphoramidate. This directly bonded coupling experiences a large increase in going to the larger ring system, reflecting the change from a pyramidal-like (sp3) nitrogen in the three-membered ring to trigonal (sp2) in the larger ring compounds and the acyclic (diethylamino)dimethylphosphoramidate, where the observed coupling is 42.2 Hz.

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Journal Journal of Organic Chemistry
Gray, G.A. (George A.), Buchanan, G.W, & Morin, F.G. (Frederick G.). (1979). 15N nuclear magnetic resonance of organophosphorus compounds. Ring size and aziridine methylation effects on 15N shifts and 15N-31P nuclear spin couplings in heterocyclic phosphoramidates. Journal of Organic Chemistry, 44(11), 1768–1771.