REACTIONS OF CHARGED SUBSTRATES .4. THE GAS-PHASE DISSOCIATION OF (4-SUBSTITUTED BENZYL)DIMETHYLSULFONIUMS AND (4-SUBSTITUTED BENZYL)DIMETHYLPYRIDINIUMS

The relative rates for the gas-phase dissociation RX(+) --> R(+) + X d egrees of five (4-Y-substituted benzyl)-dimethysulfoniums (Y = MeO, Me , H, Cl, and NO2) and 24 (4-Y-substituted benzyl)-3'-Z-pyridiniums (co mplete series for Z = CN, Cl, CONH2, and H, and 4-methoxy- and 4-nitro benzyls for Z = F and CH3CO) were measured using liquid secondary ion mass spectrometry. The Hammett plot (vs delta Delta G degrees or sigma (+)) is linear for the sulfoniums, but plots for the four pyridinium s eries have a drastic break between the 4-Cl and 4-NO2 substrates. Broi nsted-like plots for the pyridiniums show a strong leaving group effec t only for 4-nitrobenzyls. An analysis of these linear free energy rel ations with supporting evidence from semiempirical computations sugges ts that collisionally activated pyridinium substrates dissociate by tw o pathways, direct dissociation and through an ion-neutral complex int ermediate. Comparison of these results with results for the solution r eactions of some of these compounds shows that the mechanism is differ ent in the gas and solution phases. Sufficient experimental data are n ot available to assign a mechanism for dissociation to the sulfonium s eries, but computational results show characteristics of a direct diss ociative mechanism.