Isomerization of alkane molecular ions

Abstract

The appearance energies of daughter ions for the major low-energy losses of C/sub 2/H/sub 6/, CH/sub 4/, and CH/sub 3/., of pentane and methylbutane radical cations were measured. The behaviors of a variety of /sup 13/C- and /sup 2/H-labeled compounds were examined. For methylbutane, loss of C/sub 2/H/sub 6/ and CH/sub 3/. yielded ionized propene and 2-butyl cations, at their calculated thermochemical thresholds. Loss of CH/sub 4/ proceeded at an energy greater than that calculated for the production of ionized but-2-ene, methylpropene, but-1-ene, or methylcyclopropane. Ethane elimination is preceded partially (ca. 20 to 25%) by a 1 to 2 methyl shift and a concomitant 2 to 1 H shift; isotope effects associated with this reaction are discussed. For pentane, losses of CH/sub 3/. and CH/sub 4/ take place at the same energy, corresponding to the calculated threshold for production of the secondary cations; /sup 13/C labelling experiments showed that the penultimate C atoms are not lost but that C-3 is lost (46%) in these reactions, with an ease similar to terminal C atom losses (54%). /sup 2/H labelling permitted the separation of the metastable peak for methane loss into components having different kinetic energy releases attributable to the production ofmore » different (C/sub 4/H/sub 8/)/sup +/. daughter ions. Although C/sub 2/H/sub 6/ 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 CH/sub 3/. and CH/sub 4/ losses. Labelling experiments showed that the central C atom is not lost in this reaction. Detailed analysis of the observations leads to the conclusion that the CH/sub 3/., CH/sub 4/, and C/sub 2/H/sub 6/ eliminations from pentane radical cations are preceded by an isomerization to energy-rich ionized methylbutane. 2 figures, 3 tables.« less