Structure and fragmentation of [C6H13O]+ ions formed by chemical ionization

The structure and fragmentation of eight [C6H13O] + ions formed by protonation of C6H12O carbonyl compounds in the gas phase have been investigated using isotopic labeling and metastable ion studies to investigate the fragmentation reactions and collisional dissociation studies to probe ion structures. Protonated 3‐methyl‐2‐pentanone and protonated 2‐methyl‐3‐pentanone readily‐interconvert by pinacolic‐retro‐pinacolic rearrangements; the remaining six ions represent stable ion structures, although in many cases fragmentation is preceded by pinacolic‐type rearrangements. Unimolecular (metastable ion) fragmentation of the [C6H13O] + species occurs by elimination of H2O, C3H6, C4H8 and C2H4O. The last three elimination reactions appear to occur through the intermediacy of a proton‐bound complex of a carbonyl compound and an olefin, with the proton residing with the species of higher proton affinity on decomposition of the complex.