The collisionally induced dissociation of allyl and 2‐propenyl cations

Pure [CH2CHCH2]+ and {article}{empty}{document}CH₃ C⁺ = CH₂{document} ions are generated only in metastable fragmentations of [CH2CHCH2X]+˙, X=Cl, Br, I, and [CH3CXCH2]+˙, X=Br, I, respectively. For ion source generated [C3H5]+ ions there is some structural interconversion. The structure characteristic feature of their collisional activation mass spectra is the ratio m/z 27 ([C2H3]+): m/z 26 ([C2H2]+˙). For {article}{empty}{document}CH₃ C⁺ = CH₂{document} the ratio is only weakly dependent upon the translational energy of the ion. For [CH2CHCH2]+, the ratio rises sharply as translational energy is reduced, from 0.9 at 8 kV to c. 3 at 1 kV. [CH2CHCH2]+ ions generated by charge reversal of [CH2CHCH2]− show higher ratios, resulting from their lower average internal energy content. It must therefore be emphasized that [C3H5]+ ion structure assignments should only be made using reference data which apply to specific experimental conditions. [C3H5]+ daughter ion structures for a number of well‐known fragmentations have been established. The heat of formation of the 2‐propenyl cation was measured to be 969±5 kJ mol−1. Labelling experiments show that at low internal energies, allyl cations do not undergo atom randomization in c. 1–2 μs; high internal energy ions of longer lifetime (c. 8 μs) show complete atom randomization. H˙ atom loss from [13CH3CHCH2]+˙ has been shown to generate [13CH2CHCH2]+ and {article}{empty}{document}^13CH₂ C⁺ - CH₃{document} without any skeletal rearrangement.