Carbocations

The role of quantum calculations in the field of carbocations is reviewed mainly over the past 10 years. The importance of structure and energy is emphasized. New nuclear magnetic resonance results in solution and infrared spectra in the gas phase give structural insights when coupled with theory. A critical review of applications of popular theoretical methods to the prediction of hydrocarbon and carbocation energies is presented, showing where problems can be anticipated, particularly for density functional theory methods. Gas-phase stabilities of carbocations are discussed along with the interpretation of their hydride ion affinities in terms of the fundamental factors that determine stability: polarization, inductive, ring-strain, hybridization, hyperconjugation, conjugation, and aromaticity/antiaromaticity effects. Interesting new carbocation structures and new bifurcation mechanisms are featured, along with examples of the role of carbocations in petroleum chemistry and zeolite catalysis, environmental chemistry and carcinogenic polynuclear aromatic hydrocarbons, and the biosynthesis of terpenes. In all of these areas, theoretical calculations often lead the way, filling experimental gaps that can lead to proper interpretation of experiments, providing insights that lead to a fundamental understanding of experimental results, and making new predictions of unexpected structures, properties, and reaction mechanisms for carbocations. (C) 2011 John Wiley & Sons, Ltd. WIREs Comput Mol Sci 2011 1 487-508 DOI:10.1002/wcms.12