Structure and formation of gaseous [C4H5O]+ ions. III—Ions derived from unsaturated ethers

Abstract The abundant [C 4 H 5 O] + ( m / z 69) ions found in the 70 eV mass spectra of a series of acetylenic, allenylic and unsaturated cyclic ethers are shown to have the following structures: HCC $$ \mathop {\rm C}\limits^{\rm + } $$ HOCH 3 ( e ), H 2 CC—OCH 3 ( f ), ( g ) and HCCCH 2 —O $$ \mathop {\rm C}\limits^{\rm + } $$ H 2 ( h ). Of these, the cyclic ion g is the most stable: its ion enthalpy (≥ 165 kcal mol −1 ) is close to that found for the acyclic C 3 H 5 $$ \mathop {\rm C}\limits^{\rm + } $$ O isomers identified in a previous study. Evidence that these four isomeric [C 4 H 5 O] + ions are stable species with lifetimes ≥ 10 −5 s is obtained from their collisional activation spectra, the shape of the metastable peaks and the associated kinetic energy release values for the common loss of CO, thermochemical information and analysis of deuterium and carbon‐13 labelled precursor molecules. It is further shown that loss of X⋅ from ethers of the type XCCCH 2 OCH 3 involves isomerization into energy rich allenyl type ions [(X)HCCCHOCH 3 ] + ˙ . These ions undergo loss of X⋅ by simple bond cleavage, yielding, e type product ions, when the CX bond strength is relatively low (XI, Br). When XCl and especially CH 3 or H, X⋅ is only lost after rearrangement yielding the cyclic product ion g . The mechanism for this cyclization reaction is related to that proposed in a previous study for the ester→ acid isomerization in the molecular ions of the esters of α, β‐unsaturated carboxylic acids.