Rearrangement Reactions in the Electron Impact Fragmentation of Isobutene

The detailed mass spectrum of isobutene has been examined using both D and 13 C 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 C 3 H 5 + . 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 C 2 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.