The elusive HNCOH + ion characterized by a double‐collision experiment in the mass spectrometer

Previous studies have shown that the gas‐phase protonation of isocyanic acid, HNCO, exclusively yields the N‐protonated ion, the aminoformylium ion, H 2 NCO + , 1, which is thermodynamically the most stable [H 2 , C, N, O]isomer. In the present paper it is shown that the O‐protonated species, the elusive isoformylaminylium ion, HNCOH + , 2, can be generated by successive degradation of ionized N ‐methyl‐ O ‐methylcarbamate in a 3‐stage tandem (MS/MS/MS) type experiment: CH 3 NHC(O)OCH   3 + · → CH 3 NHCOH + · → HNCOH + · The species is identified from its characteristic collisional activation (CA) mass spectrum. From a comparison of the CA mass spectra of the 2 H‐labelled isotopomers DNCOH + and HNCOD + with that of the isomeric ion HDNCO + it follows that collisionally excited ions (>400 kJ/mol) of 2 may isomerize into 1.