No abstract

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.

The collisionally activated dissociation of a variety of isomeric disubstituted aromatic ions formed by ion-molecule reactions were examined in order to characterize ortho effects in closed-shell systems. Closed-shell ions of metboxyacetophenooe, hydroxyacetophenone, methoxyphenol, anisaldehyde and hydroxybenzaldehyde were formed by proton transfer, methyl addition or methyne addition by using dimethyl ether or ethylene oxide as chemical ionization reagents, and then the structures of these adducts were studied by deuterium-labelling methods and by collisionally activated dissociation techniques in a triple quadrupole mass spectrometer or a quadrupole ion trap. Typically, the metu and para isomers have qualitatively similar dissociation spectra which reflect the types of dissociation reactions observed for the corresponding monosubstituted aromatic ions. The predominant dissociation pathways of the [ M + H] + and [ M + 151 + ions are directed by the electron-withdrawing substituents, whereas the major dissociation pathways of the [M + 131' ions are related to the electron-releasing substituent. In contrast, the dissociation routes of the corresponding ortho isomers are dramatically different. This is attributed to the opportunity for functional group interactions of the ortho isomers which facilitate alternative pathways.

The llpe of bimolecuhr reactions as selective and sensitive diagnostic probes of analytically relevant molcules has undergone rapid development, especiPlly with tbe increasing availability of ion trapping techniques which allow detailed exsmiacrtioa of ion-molecule reactions. In this report, selective ion-molecule reactions of 2 , s dimethylpyrrole-y-butyrolacto~ with protoaated dimethyl ether and methoxymethylene cations (CH,OCH, +) are described. A qwdrupole ion trap and a triplequadrupole mass spectrometer are used to examine the bimolecular and dmiation reactions of y-butyrolactone and 2,5dimethylpyrrole, both of which are the building blocks of a more complex bicyclic substrate, 2,5-dimethylpyrrole-y-butyrolactone. All three compounds are protonated by ionmolecule reactions with dimethyl ether ions. The y-butyrolactoue also reacts with CH30CHz+ to produce

To elucidate the selectivity of methylene substitution reactions of monosubstituted and disubstituted oxyaromatic compounds in a low pressure quadrupole ion trap environment, the relative abundances of covalently bound and loosely bound adducts formed by ion/molecule reactions with ethylene (ET), ethylene oxide (ETOX), and dimethyl ether (DME) were compared. Adduct ions of all three reagent gases were formed in both a conventional ion source and a quadrupole ion trap and characterized by collisionally activated dissociation. For DME and ET, the covalently bound adducts formed at (M + 45)(+) and (M + 41)(+), respectively, are direct precursors to the methylene substitution product ions at (M + 13)(+). ETOX and ET do not demonstrate the same functional group selectivity for methylene substitution as previously observed for DME. This is attributed to differences in reaction exothermicities and competing reactions.