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, protonate d arene formation with alkene loss, and benzylic cation formation with alkane elimination. The dissociations are often preceded by an exchan ge between the hydrogen atoms of the chain and those of the ring, whic h can be rationalized by an interconversion between [arene alkyl catio n] and [protonated arene alkene] ion-neutral complexes. By comparing t he reactions of the metastable protonated alkyl arenes and those of th eir isomeric adducts (arene/alkyl cation) generated in the ion source by ion-molecule reaction, we show that ct-complexes [arene alkyl catio n] have a significant lifetime. By ab initio calculations, [C6H6 iso-C 3H7+] and [C6H7+ C3H6] ion-neutral complexes are shown to be energy mi nima on the potential energy surface. They correspond therefore to sta ble structures. Two methods, namely, ab initio and semiempirical calcu lations, are used to calculate the interaction energies of [arene alky l cation] complexes. The smallest stabilization energy (49 kJ/mol) cor responds to [C6H6 tert-C4H9+] and the largest (65 kJ/mol) corresponds to [p-CH3C6H4CH(3) iso-C3H7+] The [protonated arene alkene] complexes are less stabilized (ca. 25 kJ/mol). The calculations also indicate th at [arene alkyl cation] complexes have pi-structures, but do not corre spond to electron donor-acceptor (EDA) systems since, among the compon ents of their interaction energy, the charge exchange term is negligib le. Finally, these results are used to propose energy diagrams and fra gmentation pathways.