Unusual Fragmentation of 1,1,2,2,3,3-Hexamethylindan. Methyl Group Equilibration and Multi-step Skeletal Rearrangements in the (1+) Ions Prior to the Formation of t-C4H9(1+) Other Fragment Ions

Article abstract of DOI:10.1002/oms.1210270416

Based on the surprising observation of an intense C4H9(1+) (m/z 57) peak in the electron impact mass spectrum, the fragmentation of 1,1,2,2,3,3-hexamethylindan (2) was studied by mass-analysed ion kinetic energy spectrometry of its deuterium-labelled analogues.While methyl loss from ions <2>(1+)^. occurs with high selectivity (92percent) from the positions 1 and 3 without any rearrangement, ions <2-CH3>(1+) undergo complete equilibration of the five methyl groups as intact entities.Subsequent multi-step skeletal rearrangement of the <2-CH3>(1+) ions leads to formation of tert-butyl ions and to the loss of isobutene and propene, again without concomitant hydrogen exchange.Several kinetic isotope effects and also probably a thermodynamic isotope effect associated with each of these fragmentation processes have been found and their origin is discussed.The possibility of the formation of ion-neutral complexes and is considered on the basis of the labelling and reactivity pattern.