title: Use of borinium ions as probes of steric effects in gas-phase ion-molecule complexes.
abstract: 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.
authors.affiliation: Department of Chemistry and Biochemistry, University of Texas at Austin 78712, USA.
authors.forename: J J
authors.initials: JJ
authors.lastname: Isbell
authors.forename: J S
authors.initials: JS
authors.lastname: Brodbelt
mesh_terms: D001896:Boron Compounds / Q000737:chemistry*; D002138:Calibration; D002849:Chromatography, Gas; D007202:Indicators and Reagents; D013058:Mass Spectrometry; D008968:Molecular Conformation; D010455:Peptides / Q000737:chemistry; D011522:Protons; D011725:Pyridines / Q000737:chemistry
chemical_list: D001896:Boron Compounds; D007202:Indicators and Reagents; D010455:Peptides; D011522:Protons; D011725:Pyridines