The dissociation behavior of gas-phase protonated and methylated four-, five-, six-, and seven-membered ring
lactones, some with methyl substituents in various positions, has been characterized by using a quadrupole ion trap mass spectrometer and a triple quadrupole mass spectrometer. The energy dependence of collisionally activated dissociation pathways was determined by energy-resolved mass spectrometry, and the dissociation behavior of the various protonated
lactones was compared to that observed for protonated cyclic
ketones and
ethers of analogous ring size. The protonated
cyclic ethers and
ketones predominantly dissociated via
dehydration, whereas the protonated
lactones dissociated via losses of an
alkene,
ketene, and water. The dissociation behavior of the gas-phase methylated
lactones formed from ion/molecule reactions with
dimethyl ether ions was compared to the collisionally activated dissociation behavior of isomeric protonated methyl-substituted
lactones. The methylation experiments indicated that the gas-phase addition of a methyl group may dramatically alter the favored dissociation pathways when compared to the simple protonated
ions.