15N NMR Spectroscopy is one of the most powerful tools to study the structure of organic molecules.1–3δ2-pyrazolines 1 are heterocyclic hydrazones, which as a result of their cyclic structure, are stable in acidic media. Thus, their protonation has been studied by1H and13C NMR spectroscopies.4,5Pyrazolines are protonated and quaternarized at N12 and 3. It is possible to obtain the isomeric quaternary salts 4 by C-protonation of δ3-pyrazolines.6