Isomerization of Alkane Molecular Ions

Article abstract of DOI:10.1021/ja00374a030

The appearance energies of daughter ions for the major low-energy fragmentations, namely, losses of C2H6, CH4, and CH3*, of pentane and methylbutane radical cations have been measured.These reactions for ions of low internal energy content (rate constants in the range 1E5 - 1E7 s^-1) have been studied by observations of metastable peak shapes.The behaviors of a variety of 13C- and 2H-labeled compounds were also examined.Hydrogen atoms do not extensively lose their positional identity in any of these fragmentations.For methylbutane, loss of C2H6 and CH3* yielded ionized propene and 2-butyl cations, respectively, at their calculated thermochemical thresholds.Loss of CH4 proceeded at an energy greater than that calculated for the production of ionized but-2-ene, methylpropene, but-1-ene, or methylcyclopropane.The labeling experiments permitted the separation of the complex Gaussian-type metastable peak for CH4 loss into high and low kinetic energy release components, each appropriately relating to the generation of different daughter ions via competing mechanistic pathways.Large isotope effects were observed, similar in magnitude to those observed for the loss of methane from the methylpropane radical cation.Ethane elimination is preceded partially (ca. 20-25percent) by a 1-2 methyl shift and a concominant 2-1 H shift; isotope effects associated with this reaction are discussed. for pentane, losses of CH3* and CH4 take place at the same energy, corresponding to the calculated threshold for production of the secondary cations; 13C labeling experiments showed that the penultimate C atoms are not lost but that C-3 is lost (46percent) in these reactions, with an ease similar to terminal C atom losses (54percent).As with methylbutane, 2H labeling permitted the separation of the metastable peak for methane loss into components having different kinetic energy releases and attributable to the production of different (+*) daughter ions.Although C2H6 loss was observed to take place at energies down to the calculated thermochemical threshold for generation of ionized propene, the appearance energy of the metastable peak was the same as that for CH3* and CH4 losses.Labeling experiments showed that the central C atom is not lost in this reaction.Detailed analysis of the observations leads to the conclusion that the CH3*, CH4, and C2H6 eliminations from pentane radical cations are preceded by an isomerization to energy-rich ionized methylbutane.