Fragmentation of acylium ions from methyl levulinate. Isomerization processes involving carbon and oxygen atoms of both carbonyl groups

The fragmentations of the acylium ions OC + CH 2 CH 2 CO 2 CH 3 and OC + CH 2 CH 2 COCH 3 generated from methyl levulinate are governed extensively by the interaction of the two carbonyl groups. Both species eliminate a molecule of CO unimolecularly and under CID conditions. The results derived from measurements of 13 C and 18 O labelled precursors, together with kinetic energy release values, have been used to study the mechanisms. In the first of these acylium ions, both carbonyl groups are equivalent; this phenomenon can be the result of a 1,4 methoxy shift. In the second acylium ion, only the oxygen atoms change their positions; this isomerization occurs via the [MH] + of γ‐valerolactone. Some other fragmentation processes also discussed in relation to 2 H labelling are the formation of the [M COOCH 3 ] + ion and the loss of HCOOCH 3 in the collision‐induced dissociation mass spectra of the first acylium ion, and the formation of the [CH 3 CO] + ion and the loss of H 2 O for the second one.