LARGE GASEOUS ALKYLBENZENIUM IONS - THE APPARENTLY SIMULTANEOUS LOSS OF 2 MOLECULES OF BENZENE FROM PROTONATED TRI-BENZYLMETHANE AND TETRABENZYLMETHANE

The consecutive loss of two molecules of benzene from several protonat ed oligophenylisoalkylbenzenium ions has been studied by chemical ioni zation/mass analyzed ion kinetic energy (CI/MIKE) and CI/collision ind uced dissociation (CID)I MIKE spectrometry. The twofold elimination of benzene is most pronounced for protonated tribenzylmethane (1) and te trabenzylmethane (2). In the latter case, this process represents simi lar to 98% of the fragmentation, even of the metastable ions dissociat ing in the second field-free region of a double focusing mass spetrome ter. In contrast, the higher homologues of 1 (3-6) dissociate predomin antly 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-bran ched protonated alkylbenzenes. Collision (CID/MIKE) spectrometry has b een performed to elucidate the structures of the [M + H - 2 C6H6] + io ns (C10H11+ from 1 and C17H17+ from 2). The results suggest that homob enzylic and benzylic carbenium ions (indanyl- and/or tetralyl-type ion s) are formed whereas the intermediate [M + H - C6H6](+) ions represen t 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 oligophenylalkylben zenium ions has been considered. Whereas high proton mobility represen ts a mechanistically salient feature for the fast twofold loss of benz ene, the relevance of protonated arene heterodimers to the apparent si multaneity of the elimination of two benzene neutrals cannot be corrob orated. (Int J Mass Spectrom 179/180 (1998) 147-163) (C) 1998 Elsevier Science B.V.