Energetic and electron density analysis of hydrogen dissociation of protonated benzene

Abstract

The electronic structure of the benzenium cation, [C₆H₇]⁺, the simplest intermediate of electrophilic aromatic substitution reactions, was analyzed in terms of the properties of electron densities obtained from multiconfigurational quantum theoretical methods. The indirect C–H coupling constants and the physical contributions to their values were calculated and rationalized in terms of the electron delocalization between the quantum topological atoms in the molecule. The evolution of the electronic structure for the intramolecular proton migration and for the dissociation into [C₆H₆]⁺ + H, or [C₆H₅]⁺ + H₂ was also studied. The potential energy surface for intramolecular H migration has six equivalent transition states and two equivalent two-fold saddles, whereas each dissociation process occurs without the presence of any transition state. The calculated energy barriers of 9.5, 80.3 and 72.4 kcal mol⁻¹ for the intramolecular proton migration, H and H₂ eliminations, respectively, agree with experimental reports. The quantitative chemical descriptors based on the electron density of the benzenium cation provide insight on the nature of the chemical bond, including electron delocalization, and its evolution during chemical transformations of the molecule.