Loss of isobutane is the exclusive fragmentation channel of long-lived protonated tert-butyl-substituted alpha,omega-diphenylalkanes with chain lengths n = 2-12. Methane chemical ionization/mass-analysed ion kinetic energy spectrometry of the [M + H]+ ions and several deuterium-labelled tert-butyl-substituted 1,2-, 1,3- and 1,10-diphenylalkanes revealed that the reaction involves with equal probability and maximum regioselectivity a hydride ion from both the adjacent and the remote benzylic methylene groups, irrespective of the length of the aliphatic chain. The isotope effect of the hydride abstraction step was found to be same in all cases, k[C4H10]/k[C4H9D] = 1.6 +/- 0.1. A sterically restricted system, protonated 1-benzyl-4-(tert-butylbenzyl)benzene, shows the same behaviour. This implies, as a general phenomenon, the formation of intermediate ion-neutral complexes [C4H9+ Ar(CH2)(n)Ar] along with (or instead of) the corresponding pi complexes [C4H9+...Ar(CH2)(n)Ar] prior to isobutane elimination (Ar represents a phenyl or substituted-phenyl group).
