STRUCTURES AND MECHANISMS OF REACTIONS OF ISOMERIC C2H3O+ AND C2H3S+ IONS REVEALED THROUGH ION MOLECULE REACTIONS IN CONJUNCTION WITH 2D AND 3D MASS-SPECTROMETRY

Ion/molecule reactions are used to demonstrate that mass-selected C2H3O+ and C2H3S+ ions have distinctive reactivities which depend on the precursor molecule from which they are generated. Several isomers of both ions are distinguished, and the mechanisms of their reactions are elucidated. Structural characterization of the ion/molecule products is achieved in multistage (MS3) experiments. In reactions with isoprene, the acetyl cation (a) is unique in that it displays a [4 + 2+] Diels-Alder cycloaddition product while other C2H3O+ isomers, O-protonated ketene (b) and the oxiranyl cation (c), as well as the isobar C3H7+, undergo proton-transfer reactions. The proportion of the acetyl ion in m/z 43 ion mixtures is estimated by comparing the extent to which cycloaddition and proton transfer take place. On this basis, compounds with various functionalities are ordered in terms of the degree to which their m/z 43 fragments comprise the acetyl cation. Correlations are developed with the degree of enolization in the molecular ions of the precursors. Further distinction of C2H3O+ isomers a-c is achieved in reactions with methylanisoles. Ion a preferentially forms the intact adduct with these reagents while ions b and c fail to do so, their spectra being dominated by characteristic charge-exchange, proton-transfer, and adduct-fragmentation products. Ion c reacts with m- and o-methylanisole by a process which formally corresponds to CH+ addition to the neutral molecule. Reaction with p-methylanisole shows predominant formation of the CH2.+ addition product. Sequential product MS3 spectra show that methyne addition yields m- and o-methylmethoxytropylium ions, while methylene addition yields ionized 2,4-dimethylanisole. The C2H3S+ ions formed from a variety of precursors show similar product spectra upon reaction with isoprene, all displaying abundant cycloaddition products. Sequential product spectra are identical, a result that indicates formation of a common adduct. Reaction with m-methylanisole, however, allows distinction of the isomeric C2H3S+ ions and shows that the failure to distinguish these isomers by reaction with isoprene is the result of isomerization of the different cycloaddition products to a common structure. Similarly, failure to distinguish the C2H3S+ ions by previous collision-induced-dissociation experiments is interpreted to be a result of isomerization after collision, not of the existence of a single ionic population. These results provide an illustration of the detailed information on ions which is possible using ion/molecule reactions and the degree to which the structures of ion/molecule reaction products can be elucidated using multistage (pentaquadrupole) mass spectrometry.