Collision activation studies of mucoactive phthalmic acids and model cations

Collision activation experiments, using a suitable model ion, strongly support the previously suggested structure of protonated N ‐methylisoindolinone for the ionic product of H' loss from N,N ‐dimethylbenzamide upon electron impact. The cyclization process is preceded by a hidden H‐rearrangement within the amidic group. Such an H' loss, occurring also for phthalamic acids, as expected, does not take place for 2‐( N,N ‐dimethylacetamido)nicotinic acid, which likely undergoes a cyclization of N ‐methylazaisoindolinone ionic product by consecutive eliminations of H 2 O and CO.