Unusual fragmentation of 1,1,2,2,3,3‐hexamethylindan. Methyl group equilibration and multi‐step skeletal rearrangements in the [M  CH 3 ] + ions prior to the formation of t ‐C 4 H 9 + and other fragment ions

Based on the surprising observation of an intense C 4 H 9 + ( 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] + ˙ occurs with high selectivity (92%) from the positions 1 and 3 without any rearrangement, ions [2 CH 3 ] + undergo complete equilibration of the five methyl groups as intact entities. Subsequent multi‐step skeletal rearrangement of the [2 ‐ CH 3 ] + ions leads to formation of ferf‐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 [ t ‐C 4 H 9 + C 10 H 10 ] aud [ s ‐C 3 H 7 + C 11 H 12 ] is considered on the basis of the labelling and reactivity pattern.