Nitrogen NMR sudies of some 15 N labelled sydnones and related stuctures

15 N NMR studies on some signly labelled sydnones, N ‐acetylsydonimines and their hydrochlorides, as well as those on some sydnonimine hydrochlorides, show that in each case protonation takes place at the exocyclic moiety, \documentclass{article}\pagestyle{empty}\begin{document}$ - {\rm O}^ \ominus - \mathop {\rm N}\limits^ \ominus {\rm COCH}_3 \,{\rm or}\, - \mathop {{\rm NH}}\limits^ \ominus $\end{document} , respectively. Complete assignments of the nitrogen chemical shifts are possible for the labelled compounds, including the isomeric structures of N ‐alkyl– N ‐cyanomethyl– N –nitrosoamines to which, unstable, free sydnonimines are converted.