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.