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

<jats:title>Abstract</jats:title><jats:p>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 <jats:styled-content>\documentclass{article}\pagestyle{empty}\begin{document}$ {\rm CH}_3 \mathop {\rm C}\limits^ + ({\rm OH}){\rm O}\mathop {\rm C}\limits^{\rm .} {\rm H}_2 $\end{document}<jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/tex2gif-ueqn-1.gif" xlink:title="equation image" /></jats:styled-content>, in contrast, methyl propanoate molecular ions isomerize into <jats:styled-content>\documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm C}\limits^. {\rm H}_2 {\rm CH}_2 \mathop {\rm C}\limits^ + ({\rm OH}){\rm OCH}_3 $\end{document}<jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/tex2gif-ueqn-2.gif" xlink:title="equation image" /></jats:styled-content>. Metastably fragmenting methyl acetate molecular ions are known predominantly to form H<jats:sub>2</jats:sub>ĊOH together with <jats:styled-content>\documentclass{article}\pagestyle{empty}\begin{document}$ {\rm CH}_3 - \mathop {\rm C}\limits^ + = {\rm O} $\end{document}<jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/tex2gif-ueqn-3.gif" xlink:title="equation image" /></jats:styled-content>, whereas ionized methyl propanoate largely yields H<jats:sub>3</jats:sub>CO˙ together with <jats:styled-content>\documentclass{article}\pagestyle{empty}\begin{document}$ {\rm CH}_3 {\rm CH}_2 - \mathop {\rm C}\limits^ + = {\rm O} $\end{document}<jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/tex2gif-ueqn-4.gif" xlink:title="equation image" /></jats:styled-content>. The observations were explained in terms of the participation of different distonic molecular ions. The enol form of ionized methyl acetate generates substantially more H<jats:sub>3</jats:sub>CO˙ in admixture with H<jats:sub>2</jats:sub>ĊOH 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 <jats:italic>ab initio</jats:italic> calculations at a high level of theory would be required to establish detailed reaction mechanisms.</jats:p>