Unusual fragmentation of 1,1,2,2,3,3‐hexamethylindan. Methyl group equilibration and multi‐step skeletal rearrangements in the [M  CH3]+ ions prior to the formation of t‐C4H9+ and other fragment ions

Abstract

AbstractBased on the surprising observation of an intense C4H9+ (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  CH3]+ undergo complete equilibration of the five methyl groups as intact entities. Subsequent multi‐step skeletal rearrangement of the [2 ‐ CH3]+ 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‐C4H9+ C10H10] aud [s‐C3H7+ C11H12] is considered on the basis of the labelling and reactivity pattern.