Margaret S. Lin

Rearrangement Reactions in the Electron Impact Fragmentation of Isobutene

The detailed mass spectrum of isobutene has been examined using both D and 13C labelling. It is shown that at low average internal energies of the molecular ion complete randomization of hydrogens and of carbons occurs prior to fragmentation to form C3H5+. As the average internal energy of the molecular ion increases (by increasing the ionizing electron energy) the extent of both carbon and hydrogen randomization decreases. Carbon scrambling is complete in the molecular ion prior to fragmentation to form C2 ions under all conditions studied. The results are consistent with a skeletal isomerization of the isobutene molecular ion by a mechanism involving a series of 1,3 ring closures to form methylcyclopropane type ions.

Mass spectrometry of compounds of the type [Fe(CO)2(η-C5H5)R](R = alkyl)

The electron-impact mass spectra of a series of compounds of the type [Fe(CO)2(η-C5H5)R](R = alkyl) have been obtained. Three major fragmentation routes commence with loss of alkyl (R), alkene (R–H), or carbonyl groups. Particularly intense peaks are also formed by loss of one, two, or three molecules of H2 from the [M– 2CO]+ ions, leading to species which presumably contain η3-allylic and η5-pentadienyl moieties.

Energy dependence of the fragmentation of the molecular ions of some branched C7H14 olefins

Abstract Breakdown graphs have been established for ten branched C7H14 olefins using charge-exchange techniques. For 2-methyl-1-hexene, 5-methyl-1-hexene, 2,3-dimethyl-1-pentene and 2,4-dimethyl-1-pentene, C5H10+· is the dominant primary fragmentation product at low internal energies and is produced at the approximate thermochemical threshold. While formation of this product is favoured energetically, it is strongly disfavoured in terms of frequency factor and cannot compete at higher internal energies with alternative fragmentation reactions. For 3-methyl-1-hexene and 4-methyl-1-hexene, C5H10+· is not formed at the thermochemical threshold but is produced in lower yield at higher internal energies in competition with other fragmentation reactions. Although C5H10+· is the most abundant product of decomposition of metastable C7H14+· ions arising from 2-methyl-2-hexene and 2-methyl-3-hexene, it is not observed with significant abundance in the breakdown graphs: this suggests that the long-lived molecular ions have undergone rearrangement. The C6H11+ fragment ion is not formed at the thermochemical threshold except for 2,3-dimethyl-2-pentene and 2,4-dimethyl-2-pentene where it is the dominant primary fragment ion in the breakdown graphs as well as the dominant product of metastable ion fragmentation. The breakdown graphs in combination with approximate internal energy distributions derived from He(I) photoelectron spectra adequately rationalize the observed electron impact mass specta.