Gas phase ion chemistry of methyl acetate, methyl propanoate and their enolic tautomers. An experimental approach

From a combination of isotopic substitution, time‐resolved measurements and sequential collision experiments, it was proposed that whereas ionized methyl acetate prior to fragmentation rearranges largely into {article}{empty}{document}CH₃ C^ + ( OH) O C. H₂{document} , in contrast, methyl propanoate molecular ions isomerize into {article}{empty}{document}C^. H₂ CH₂ C^ + ( OH) OCH₃{document} . Metastably fragmenting methyl acetate molecular ions are known predominantly to form H2ĊOH together with {article}{empty}{document}CH₃ - C^ + = O{document} , whereas ionized methyl propanoate largely yields H3CO˙ together with {article}{empty}{document}CH₃ CH₂ - C^ + = O{document} . The observations were explained in terms of the participation of different distonic molecular ions. The enol form of ionized methyl acetate generates substantially more H3CO˙ in admixture with H2ĊOH than the keto tautomer. This is ascribed to the rearrangement of the enol ion to the keto form being partially rate determining, which results in a wider range of internal energies among metastably fragmenting enol ions. Extensive ab initio calculations at a high level of theory would be required to establish detailed reaction mechanisms.