Functional group-selective ion-molecule reactions of ethylene glycol and its monomethyl and dimethyl ethers

The selective methylation and methylene substitution reactions of dimethyl ether ions with ethylene glycol, ethylene glycol monomethyl ether, and ethylene glycol dimethyl ether were investigated in a quadrupole ion trap mass spectrometer. Whereas the reactions of ethylene glycol and ethylene glycol monomethyl ether with the methoxymethylene cation 45⁺ gave only [M + 13]⁺ product ions, the reaction of ethylene glycol dimethyl ether with the same reagent ion yielded exclusively [M + 15]⁺ ions. The relative rates of formation of these products and those from competing reactions were examined and rationalized on the basis of structural and electronic considerations. The heats of formation for various relevant species were estimated by computational methods and showed that the reactions leading to the [M + 13]⁺ ions were more energetically favorable than those leading to the [M + 15]⁺ products for cases in which both reactions are possible. Finally, the collision-induced dissociation behavior of the [M + H]⁺, [M + 13]⁺, and [M + 15]⁺ ions indicated that the and [M + H]⁺ rons dissociated by analogous pathways and were thus structurally similar, whereas the [M + 13]⁺ ions possessed distinctly different structural characteristics.