PUB - Publikationen an der Universität Bielefeld

The fragmentation of metastable 6-methylfulvenium and 6,6-dimethylfulvenium ions, generated by gas-phase protonation of the neutral hydrocarbons in the CI(CH4) plasma, has been investigated by means of mass-analyzed kinetic energy (MIKE) spectrometry. The C7H9+ ions formed from 6-methylfulvene lose H-2 and CH4, and the C8H11+ ions formed from 6,6-dimethylfulvene lose H-2, CH4 and C2H4 in relative amounts and with kinetic energy release characteristics that are reminiscent of the fragmentation behavior of toluenium and xylenium ions, respectively. The isomerization reactions preceding the fragmentation of long-lived protonated 6,6-dimethylfulvene, C8H11+, have been studied in detail by C-13- and H-2-labelling of this cycloolefin and analysis of the isotope distributions observed in the MIKE spectra of the [M + H - methane](+) and [M + H ethene](+) ions, in particular. Composite scrambling was found to occur prior to both of these fragmentation reactions, that is, metastable C8H11+ ions form two populations that react through different isomerization channels prior to loss of the same neutral fragment. The loss of CH4 is largely (95%) C-specific but only CH3-specific for ca. 50% of the ions, whereas the other 50% fraction of the ions suffer complete H scrambling. The loss of C2H4 is much less specific: it takes place from a 70-80% fraction of C8H11+ ions in which one of the two methyl groups is retained intact, whereas the other is involved in a fast C- and H-scrambling of the remaining C7H8 backbone, and also from a 20-30% fraction in which all of the eight C- and the eleven H atoms undergo complete scrambling. The composite scrambling behavior observed through expulsion of ethene is almost identical to that found previously upon protonation of para-xylene by CI(CH4). Therefore, it is suggested that the C8H11+ ions generated by protonation of 6,6-dimethylfulvene readily isomerize to 1, 1 -dimethylbenzenium and the related xylenium ions. The minor (20-30%) fraction of the latter ions undergoes a further, reversible ring expansion to methyldihydrotropylium ions before re-contracting the seven-membered ring to yield ethylbenzenium ions; the major (70-80%) fraction does so irreversibly before re-contraction to ethylbenzenium ions, from which ethene is expelled eventually. (C) 2007 Elsevier B.V. All rights reserved.