The Gas-Phase Reactions of the Methoxymethyl Cation with Aldehydes and Ketones

Abstract

The results of both mass-analysed ion kinetic energy spectroscopy and Fourier transform ion cyclotron resonance experiments involving reactions of the methoxymethyl cation CH₃OCH₂⁺ with a variety of aldehydes and ketones are reported. With ketones, the reaction yields a covalent complex whose dissociation either gives back the CH₃OCH₂⁺ cation or leads to a methyl cation transfer to the ketone. Except for CH₂O, a third pathway is open with aldehydes. A hydride transfer to CH₃OCH₂⁺ yields a RCO⁺ acylium product ion. The branching ratio of these three pathways strongly depends on the structure of the aldehyde. Ab initio calculations confirm that the results can be explained by the interconversion of covalent structures and electrostatic complexes on the potential energy surface.