ELECTRON-IMPACT-INDUCED FRAGMENTATION OF LONG-LIVED C14H14-CENTER-DOT-DIPHENYLETHANE( IONS FROM DIBENZYL SULFONE AND 1,2)

Gaseous dibenzyl sulphone radical cations (1(.+)) expel SO2, forming C 14H14.+ ions, whose fragmentation is identical with that of the radica l cations of 1,2-diphenylethane (2(.+)). Metastable ions [1 - SO2](.+) and 2(.+) eliminate predominantly benzene to give C8H8.+, in competit ion with the well-known benzyl cleavage. The [M - SO2](.+) ions from d euterium-labelled isotopomers of 1 react in exactly the same way as 2( .+) ions bearing the label in corresponding positions (e.g. C6H5CD2SO2 CD2C6H5.+ and C6H5CD2CD2C6H5.+). Benzene loss is initiated by migratio n of a benzylic H atom, but does not occur by simple I,2-elimination; rather, the C6H6 molecule incorporates an H atom from the other ring. Both fragmentation reactions are preceded by slow interannular hydroge n exchange, which involves preferentially the ortho positions of the r ings; however, this exchange is coupled with a relatively fast intra-a nnular hydrogen ring-walk by which the hydrogens at the meta and para positions of the rings participate in the interannular exchange. The d ata suggest that the extrusion of SO2 from ionized dibenzyl sulphone g enerates a complex of a benzyl cation and a benzyl radical, which reco mbine by bond formation between the benzylic alpha carbon atoms, but p ossibly also by electrophilic attack of C6H5CH2+ at the various sites of C6H5CH2.. (C) 1997 Elsevier Science B.V.