Use of borinium ions as probes of steric effects in gas-phase ion-molecule complexes.

The relative free energies of binding of the dimethoxy borinium ion to several substituted pyridines were determined using the equilibrium method and were found to correlate well with relative gas-phase basicities except in the case in which different sterics were present near the site of cation attachment. Dimethoxy borinium ion binds to 3,5-dimethylpyridine and 3,4-dimethylpyridine 2.5+0.4 and 3.5+0.3 kcal/mole more strongly than it does to 3-methylpyridine. 2,4-Dimethylpyridine shows a 2.0+0.4 kcal/mole preference over 2-methylpyridine for dimethoxy borinium ion attachment. This difference is in agreement with the difference in gas-phase basicities of these two compounds, suggesting the borinium ion binds to the same position as the proton. 2,3-Dimethylpyridine deviated from this trend, with its borinium ion complex being 1.5+0.4 kcal/mole less stable than that of 3-methylpyridine.

Effects of functional group interactions on the gas-phase methylation and dissociation of acids and esters.

Functional group interactions have been observed to affect gas-phase ion-molecule chemistry in a quadrupole ion trap mass spectrometer. Gas-phase methylation and collisionactivated dissociation reactions of a series of related acids and esters allows an evaluation of the structural factors that influence reactivity and functional group interactions of these compounds. Examination of the [M+H](+) or [M+15](+) product ions by collision-activated dissociation has provided insight into the conformations from which diacids and diesters undergo electrophilic addition. Collision-activated dissociation has provided not only more detailed information on the structures of the ions, but also the data necessary for confident mechanistic interpretation. Labeling studies were done to probe fragmentation pathways. Upon activation of the [M+CD3](+) products of dimethyl maleate and dimethyl succinate, formed from reaction of the neutrals with CD3OCD 2 (+) ions, a rapid interfunctional group methyl transfer causes scrambling of the methyls prior to elimination of dimethyl ether or methanol. The [M+15](+) ions of dimethyl maleate are believed to lose dimethyl ether through a rate-determining 1,6-methyl transfer, whereas the [M+15](+) ions of dimethyl succinate eliminate methanol through a rate-determining 1,5-proton transfer.