The distinctive behavior of isomeric methyl ethyl mixed esters of 2-methylmaleic acid upon electron ionization

Isomeric mixed methyl ethyl esters of 2-methylmaleic acid (citraconic acid) show different electron ionization (EI) mass spectra. Methyl radical is exclusively lost from the M+ ion of one isomer ((Z)-methyl 3-ethoxycarbonyl-2-butenoate), while the elimination of methanol is more efficient in the other ((Z)-ethyl 3-methoxycarbonyl-2-butenoate). Mechanistic pathways have been proposed for these two specific reactions based on the results of deuterium labeling and collision-induced dissociation (CID) studies. The initial step in both fragmentation processes is the hydrogen transfer from position 2 of the ethoxy group to the oxygen atom of the adjacent carbonyl. The proposed ion structures and mechanistic pathways were supported by theoretical calculations using hybrid B3LYP density functional (DFT) method with the 6-31G(d,p) basis set.