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

Abstract

AbstractThe fragmentations of the acylium ions OC+CH2CH2CO2CH3 and OC+CH2CH2COCH3 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 13C and 18O 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 [MH]+ of γ‐valerolactone. Some other fragmentation processes also discussed in relation to 2H labelling are the formation of the [M  COOCH3] + ion and the loss of HCOOCH3 in the collision‐induced dissociation mass spectra of the first acylium ion, and the formation of the [CH3CO]+ ion and the loss of H2O for the second one.