The collisionally induced dissociation of allyl and 2‐propenyl cations

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