Structure and formation of gaseous [C4H5O]+ ions. I—isomeric ions of the type C3H5$$\mathop {\rm C}\limits^ + =\!= $$O

Abstract 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 $$\mathop {\rm C}\limits^ + =\!= $$ O isomers CH 2 C(CH 3 ) $$\mathop {\rm C}\limits^ + =\!= $$ O, CH 2 CHCH 2 $$\mathop {\rm C}\limits^ + =\!= $$ O, CH 3 CHCH $$\mathop {\rm C}\limits^ + =\!= $$ O and ▹ $$\mathop {\rm C}\limits^ + =\!= $$ 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 $$\mathop {\rm C}\limits^ + =\!= $$ O ions generated from a series of precursor molecules that the CH 2 CH(CH 3 ) $$\mathop {\rm C}\limits^ + =\!= $$ O‐ and CH 2 CHCH 2 $$\mathop {\rm C}\limits^ + =\!= $$ 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 CHCH $$\mathop {\rm C}\limits^ + =\!= $$ O‐type ions is proposed to occur via a rate determining 1,2‐H shift to yield 2‐propenyl cations.