PUB - Publikationen an der Universität Bielefeld

The consecutive loss of two molecules of benzene from several protonated oligophenylisoalkylbenzenium ions has been studied by chemical ionization/mass analyzed ion kinetic energy (CI/MIKE) and CI/collision induced dissociation (CID)I MIKE spectrometry. The twofold elimination of benzene is most pronounced for protonated tribenzylmethane (1) and tetrabenzylmethane (2). In the latter case, this process represents similar to 98% of the fragmentation, even of the metastable ions dissociating in the second field-free region of a double focusing mass spetrometer. In contrast, the higher homologues of 1 (3-6) dissociate predominantly or almost exclusively by single benzene loss. The mechanism and origin of the unprecedented, and apparently simultaneous, expulsion of two neutral arenes from alkylbenzenium ions is discussed in terms of the competition of cyclization and 1,2-H and 1,2-C shifts in beta-branched protonated alkylbenzenes. Collision (CID/MIKE) spectrometry has been performed to elucidate the structures of the [M + H - 2 C6H6] + ions (C10H11+ from 1 and C17H17+ from 2). The results suggest that homobenzylic and benzylic carbenium ions (indanyl- and/or tetralyl-type ions) are formed whereas the intermediate [M + H - C6H6](+) ions represent arenium ions, viz. protonated 2-benzylindane (7) and protonated 2,2-dibenzylindane (8). Protonated 2,2'-spirobiindane (9) can be excluded as the product of twofold benzene loss from 2. The role of the highly mobile protons and ion/molecule complexes in large oligophenylalkylbenzenium ions has been considered. Whereas high proton mobility represents a mechanistically salient feature for the fast twofold loss of benzene, the relevance of protonated arene heterodimers to the apparent simultaneity of the elimination of two benzene neutrals cannot be corroborated. (Int J Mass Spectrom 179/180 (1998) 147-163) (C) 1998 Elsevier Science B.V.