Metastable ion studies. III—composite metastable peaks in fragmentations of some small hydrocarbon cations

Composite metastable peaks are generated in the unimolecular fragmentations (i) [C3H5]+ → [C3H3]+ + H2 (flat‐top upon flat‐top) and (ii) [C4H9]+ → [C3H5]+ + CH4 (flat‐top and gaussian). The measurement of appearance potentials and kinetic energy releases lead us to conclude, in agreement with earlier proposals, that in (i) the components can arise from the generation of the isomeric cyclopropenium and propargyl daughter cations. In (ii) the components are proposed to arise from the fragmentation of tert‐ and sec‐butyl cations yielding allyl as the common daughter ion. The composite peak observed in the fragmentation (iii) [C3H4]+· → [C3H3]+ + H· is shown to be present only if the decomposing molecular ion is large enough to also produce [C6H8]2+ ions. The second component in (iii) then arises from the reaction [C6H8]2+ → [C6H6]2+ + H2.