Charge localization in molecule-ions upon electron-impact. Mass spectra of some substituted 1-carbamoyl- and 1-thiocarbamoyl-2-pyrazolines and related compounds

The mass spectra of some substituted 2-pyrazolines and a pyrazolidine are discussed. With the exception of III which favours McLafferty rearrangements prior to fragmentation, I, IV and V fragment to give isocyanate or isothiocyanate radicals and ions with structures corresponding to their heterocyclic molecular compounds. These results which can be explained in the light of the concept of charge localization at preferential sites, are in good agreement with the ionization potential studies on substituted ureas and thioureas.⁵ Other compounds show simple straight-forward fragmentations upon electron-impact.