Structure and formation of some [C4H5O2]+ ions generated by radical losses in pseudo simple cleavage reactions

Abstract Characterization of some [C 4 H 5 O 2 ] + ions in the gas phase using their collisional activation mass spectra shows that the isomeric ions $$ {\rm CH}_3 {\rm OCH = CH} - \mathop {\rm C}\limits^ + {\rm = O,} $$ and $$ {\rm HC} \equiv {\rm C} - \mathop {{\rm C}({\rm OH}){\rm OCH}_3 }\limits^ + $$ are stable for t ⩾10 −5 s. Of these, ions of structure were generated by the site specific gas phase protonation of γ‐crotonolactone with isobutane or methanol as chemical ionization reagent gases. These results and those derived from measurements on some 2 H, 13 C and 18 O labelled [C 4 H 5 O 2 ] + product ions, were used to study the mechanisms of unimolecular radical elimination reactions, viz. (1) loss of CH 3 ˙ from [ trans ‐methyl crotonate], (2) loss of H˙ from [methyl acrylate] + ˙, (3) loss of H˙ from [cyclopropane carboxylic acid] + ˙ and (4) loss of CH 3 ˙ from [1,3‐dimethoxypropyne] + ˙. It is concluded that none of these losses occur by simple bond cleavage. Mechanisms are presented which account for the observation that the first three reactions yield product ions of structure whereas the ions generated by reaction (4) have structure $$ {\rm CH}_3 {\rm OCH = CH} - \mathop {\rm C}\limits^ + {\rm = O}{\rm .} $$ . It is further proposed that a minor fraction of the [M‐CH 3 ] + ions from ionized trans ‐methyl crotonate is generated via a rearrangement process which yields ions of structure $$ {\rm CH}_3 {\rm OCH = CH} - \mathop {\rm C}\limits^ + {\rm = O}{\rm .} $$ .