USE OF BORINIUM IONS AS PROBES OF STERIC EFFECTS IN GAS-PHASE ION-MOLECULE COMPLEXES

The relative free energes of binding of the dimethoxy borinium ion to several substituted pyridines were determined using the equilibrium me thod and were found to correlate well with relative gas-phase basiciti es except in tile case in which different sterics were present near th e site of cation attachment. Dimethoxy borinium ion binds to 3,5-dimet hylpyridine 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 dimetho xy borinium ion attachment. This difference is in agreement with the d ifference in gas-phase basicities of these two compounds, suggesting t he borinium ion binds to the same position as the proton. 2,3-dimethyl pyridine deviated from this trend, with its borinium ion complex being 1.5+0.4 kcal/mole less stable than that of 3-methylpyridine.