Gas-phase Intramolecular Benzyl–benzyl Interactions in Protonated Dibenzyl Derivatives Containing Benzyl–oxygen, –sulfur and –nitrogen Bonds

Protonated molecules of a variety of benzyl diethers, diesters and ether-esters, produced by chemical ionization (CI), undergo a unique rearrangement yielding relatively abundant m/z 181 C14H13+ ions, both in the ion source and under collision-induced dissociation (CID) conditions. This highly general rearrangement involves an intramolecular C-C bond formation between the two benzyl groups, and the resulting C14H13+ ions have been shown by the analysis of their CID spectra to be an almost equimolar mixture of isomeric alpha-omicron-tolylbenzyl, alpha-p-tolylbenzyl and p-benzylbenzyl cation structures in all cases. This structural information suggests that this process may be viewed as gas-phase aromatic substitution of the non-charged benzoxy group by the benzyl cation originating from the protonated ether function involving a series of pi- (and/or ion-neutral) and sigma-complexes. The extent of this rearrangement process strongly depends on the nature of the benzyl bond heteroatoms. Tt is dramatically suppressed in the MH+ ions of benzyl disulfides and absent in diamines, diamides and amino-amides. The different behaviour of the O-derivatives vs. S- and N-analogues is explained in terms of the energies of the benzyl-XH+ bond heterolytic cleavages, which have been shown to increase in the order: O < S < N.