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Gas phase aromatic bromination and iodination by positive bromine or iodine species resulting from the nuclear decay of 80mBr(IT)80Br and 125Xe(EC)125I, respectively, have been studied in simple aromatic systems of the type C6H5X (X = F, Cl, Br, CH3). At high Aror Xe-moderator concentrations electrophilic substitution reactions of thermal positive halogen ions can be observed. The positional selectivity of the unsolvated Br+ or 1+ ions is very low by solution chemistry standards; not only hydrogen substitution but also replacement of the substituent X takes place. In the case of Br+ considerable rearrangement occurs in Br-for-H substitution in fluorobenzene, which increases with increasing concentration of Ar and decreasing total pressure. The results can be rationalized on the basis of excitation isomerization, which competes with collisional stabilization and proton transfer after the formation of an excited arenonium ion. For both bromine and iodine ions the ease of X replacement increases relative to that of hydrogen replacement from fluorobenzene to bromobenzene, and a concomitant increase of the relative amount of ortho substitution is observed. The formation of an aryldihalonium precursor is discussed.

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