Mass spectra of 3(2H)‐cinnolinone and 2‐substituted derivatives

The mass spectrum of 3(2H)‐cinnolinone indicates that it undergoes fragmentation in a fashion similar to other heterocyclic systems containing two vicinal nitrogens. The initial fragmentation, loss of CO, gives a 1H‐indazole radical cation, as was shown by deuterium labelling and metastable ion spectra. Four 2‐substituted‐3(2H)‐cinnolinones were also studied and it was found that their fragmentation patterns are highly influenced by the substituent groups.