Structure and formation of gaseous [C 4 H 5 O] + ions. I—isomeric ions of the type C 3 H 5 \documentclass{article}\pagestyle{empty}\begin{document} $\mathop {\rm C}\limits^ + =\!= $\end{document} O

Characterization of [C 4 H 5 O] + ions in the gas phase using their collisional activation spectra shows that the four C 3 H 5 \documentclass{article}\pagestyle{empty}\begin{document} $\mathop {\rm C}\limits^ + =\!= $\end{document} O isomers CH 2 C(CH 3 ) \documentclass{article}\pagestyle{empty}\begin{document} $\mathop {\rm C}\limits^ + =\!= $\end{document} O, CH 2 CHCH 2 \documentclass{article}\pagestyle{empty}\begin{document} $\mathop {\rm C}\limits^ + =\!= $\end{document} O, CH 3 CHCH \documentclass{article}\pagestyle{empty}\begin{document} $\mathop {\rm C}\limits^ + =\!= $\end{document} O and ▹ \documentclass{article}\pagestyle{empty}\begin{document} $\mathop {\rm C}\limits^ + =\!= $\end{document} O are stable for ≥ 10 −5 s. It is concluded further from the characteristic shapes for the unimolecular loss of CO from C 3 H 5 \documentclass{article}\pagestyle{empty}\begin{document} $\mathop {\rm C}\limits^ + =\!= $\end{document} O ions generated from a series of precursor molecules that the CH 2 CH(CH 3 ) \documentclass{article}\pagestyle{empty}\begin{document} $\mathop {\rm C}\limits^ + =\!= $\end{document} O‐ and CH 2 CHCH 2 \documentclass{article}\pagestyle{empty}\begin{document} $\mathop {\rm C}\limits^ + =\!= $\end{document} O‐type ions dissociate over different potential surfaces to yield [allyl] + and [2‐propenyl] + [C 3 H 5 ] + product ions respectively. Cyclopropyl carbonyl‐type ions lose CO with a large kinetic energy release, which points to ring opening in the transition state, whereas this loss from CH 3 CHCH \documentclass{article}\pagestyle{empty}\begin{document} $\mathop {\rm C}\limits^ + =\!= $\end{document} O‐type ions is proposed to occur via a rate determining 1,2‐H shift to yield 2‐propenyl cations.