Theoretical perspectives on carbocation chemistry from energy decomposition analysis

Abstract

Understanding carbocation formation is a central concern for all chemical sciences. The widely accepted explanation in terms of inductive/field and delocalization effects is based on quantities that are not straightforwardly computed in popular electronic structure methods. This work reports an alternative approach to the carbocation formation problem based on energy decomposition analysis, more specifically, CMOEDA. The order of stability for carbocations formation was successfully accounted in terms of the energy components. The focus of the analysis shifts from the product of the reaction, i.e., the carbocation itself, to the reactant neutral molecule. Notably, exchange repulsions are the largest energy contribution to increase carbocation stability in the order methyl, primary, secondary and tertiary. Polarization (orbital relaxation) plays a secondary role. Insertion of bulky groups increases the repulsion with the incipient anion (a hydride ion) and decreases the strength of the C–H bond. This pattern is confirmed for several other hydrocarbon cases. Additional systems like halomethanes, amino- and nitro-derivatives are also described.