Radical cations are reactive intermediates of many important organic and inorganic reactions involving singleelectron transfer or redox phenomena. In the gas phase these radical cations are prepared easily by one-electron oxidation by electron impact, and gas-phase ion-trap techniques can be used conveniently to study the course and the kinetics of their ion/molecule reactions. The results of these investigations uncover many details of the mechanisms of the reactions of these interesting species. This talk will present specifically the results of a detailed study of the reactions of two types of radical cations: the substitution of the unsaturated radical cations of haloarenes and haloalkenes by nucleophiles and the ligand exchange reactions of the radical cations of EX₃ (E = As, Sb, Bi; X = Cl, I).

It has been shown before that the radical cations of halobenzenes react with ammonia and methylamine by substitution of the halo substituent via an addition/elimination mechanism [1,2]. These investigations have been extended to the reactions of mono- and di-chloro and bromo ethene radical cations with ammonia, methylamine and methanol. The results show that again substitution of the halogen substituent occurs via an addition/elimination process, but that the mechanism of the reaction is more complicated and involves a rearrangement step by a 1,2-shift of the nucleophile in the primary distonic ion generated by addition. These experimental observations have been supported by ab initio calculations of the reaction energy profile and by RKKM calculations.

The radical cations of EX₃ (X = I, Cl) of the heavy elements As, Sb and Bi of the main group 15 react smoothly with arenes by exchange of a halogen ligand. In particular, the exchange of an iodo ligand of AsI₃^+· by benzene and the further reactions of the resulting complex [AsI₂.C₆H₆]⁺ were investigated. The reactions with arenes or with n-bases results in an efficient exchange of the benzene ligand, but never in the substitution of another iodo ligand. The affinity of the ligands to AsI₂⁺ parallels their proton affinity. The bond dissociation energies of EI₂⁺ (E = As, Sb, Bi) in some complexes were determined by energy resolved collisional activation of the dissociation. Further, the complex [AsI₂·C₆H₆]⁺ can be clearly distinguished from the isomeric phenylarsonium ion C₆H₅AsH⁺I₂. These observations will be discussed with respect to the reactivity of EX₃^+· and to the structure(s) of the complex [EX₂·arene]⁺.

1. D. Thölmann, H.-F. Grützmacher, J. Am. Chem. Soc. 1992, 115, 3281.
2. D. Thölmann, H.-F. Grützmacher, Int. J. Mass Spectrom. Ion Processes, 1992, 117, 415.