Ion-neutral complexes of protonated alkylbenzenes: Experimental and theoretical studies

In the gas phase, the unimolecular reactions of metastable protonated alkyl arenes lead to alkyl cation formation with arene loss, protonated arene formation with alkene loss, and benzylic cation formation with alkane elimination. The dissociations are often preceded by an exchange between the hydrogen atoms of the chain and those of the ring, which can be rationalized by an interconversion between [arene alkyl cation] and [protonated arene alkene] ion-neutral complexes. By comparing the reactions of the metastable protonated alkyl arenes and those of their isomeric adducts (arene/alkyl cation) generated in the ion source by ion-molecule reaction, we show that π-complexes [arene alkyl cation] have a significant lifetime. By ab initio calculations, [C₆H₆ iso-C₃H₇⁺] and [C₆H₇⁺ C₃H₆] ion-neutral complexes are shown to be energy minima on the potential energy surface. They correspond therefore to stable structures. Two methods, namely, ab initio and semiempirical calculations, are used to calculate the interaction energies of [arene alkyl cation] complexes. The smallest stabilization energy (49 kJ/mol) corresponds to [C₆H₆ tert-C₄H₉⁺] and the largest (65 kJ/mol) corresponds to [p-CH₃C₆H₄CH₃ iso-C₃H₇⁺]. The [protonated arene alkene] complexes are less stabilized (ca. 25 kJ/mol). The calculations also indicate that [arene alkyl cation] complexes have π-structures, but do not correspond to electron donor-acceptor (EDA) systems since, among the components of their interaction energy, the charge exchange term is negligible. Finally, these results are used to propose energy diagrams and fragmentation pathways.