Proton affinities and gas-phase basicities: theoretical methods and structural effects

The inherent effects of molecular structure on acid-base behavior have been a topic of debate for over 30 years. Three of the models developed to probe these effects are reviewed in this article. Each model is described briefly and the results from its application to representative systems are summarized. The first model was proposed by Taft and coworkers and subdivides intrinsic substituent effects on proton-transfer equilibria into field effects, polarizability effects and resonance effects. Various techniques used to factor out the individual contributions of these effects are described. The importance of intramolecular hydrogen bonding in determining proton affinity is also discussed. The second model was proposed by Maksic and Vianello and separates the protonation process into three sequential steps in which an electron is removed from the base, the electron is captured by the proton, and a homolytic chemical bond is formed between the radical cation and hydrogen atom. The third model was proposed by Perez and coworkers and analyzes substituent effects on proton affinity with the aid of global and local descriptors of reactivity used in conjunction with the hard and soft acids and bases principle. Several of the composite methods designed to calculate accurate thermochemical data are also discussed briefly. (C) 2003 Elsevier Science B.V. All rights reserved.