Correlations between Core Photoionization Energies (EB1s) and Gas-Phase Basicity. A General Method for Determining Sites of Protonation and Intramolecular Ion Reorganization Energies
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J. M. Buschek, F. S. Jørgensen, R. S. Brown
Based on earlier studies which showed that a correlation between 1s core electron binding energies and gas-phase basicity is expected to hold if both the site of photoemission and H+attachment are the same and large geometry changes do not occur on protonation, the present study focuses on utilizing such correlations to determine ion structure. For several bifunctional amines, not only can the site of protonation be determined with certainty but a semiquantiative assessment of the energy preference for that site can be obtained. For example, even though the intrinsic basicities of (CH3)2NH and pyridine are the same within experimental error, covalent attachment of the two groups to form 2-, 3-, or 4-dimethylaminopyridine generates species in which N-protonation is favored by 13.5, 11.7, and 21.1 kcal/mol, respectively, over substituent protonation. Utilizing the X-ray (PES) methodology, we have extended the functional group classes for which the site of protonation can be determined to include amines, pyridines, carbonyl-containing species, and nitriles. Also presented is a further application of the coupled use of X-ray PES and gas-phase basicity data to determine the energetics of internal solvation of a bifunctional molecule which on protonation reorganizes geometrically to stabilize the ion state by internal H-bonding. X-ray PES, because it is a fast technique, is an ideal method for determining how the electronic (inductive and polarization) effects of a remote substituent influence the intrinsic basicity of a molecule in its ground-state geometry. The difference between this hypothetical intrinsic basicity and the experimentally determined value reflects the energy associated with nuclear reorganization in the ion state.
|Journal||Journal of the American Chemical Society|
|Number of pages||7|
|Publication status||Published - 1 Jan 1982|