The site of gas phase cation attachment. The protonation, methylation and ethylation of morpholine, thiomorpholine and 1,4‐thioxane

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Abstract
The attachment of gaseous positive ions ([H]+, [CH3]+ and [C2H5]+) to morpholine, thiomorpholine and 1,4‐thioxane, through chemical ionization, has been studied by collision spectroscopy. The daughter ion spectra of the ion/molecule reaction products were compared to those of model ions, generated by fast‐atom bombardment of corresponding quaternary ammonium salts, in order to determine the preferred site of reaction for the protonation and alkylation of these multifunctional nucleophilic compounds. For novel entities with no model precursors, the site of cation attachment was postulated on the basis of characteristic fragmentations and trends established by the study of other bifunctional heterocycles. The site of protonation followed predicted trends in proton affinity differences for the various heteroatoms (N>S>O), and the alkyl ion reactivities followed differences in electronegativity or nucleophilicity (S>N>O).