THE REACTIONS OF POSITIVE AND NEGATIVE HALOGEN IONS WITH CL2 AND BR2

A selected ion flow tube study has been carried out at 300 K of the reactions of some atomic and molecular positive and negative halogen ions with Cl2 and Br2 from which the rate coefficients k and ion product distributions have been determined. For the energetic F+ ion reactions, dissociative charge transfer is the dominant process, while for the Cl+ ions, only nondissociative charge transfer occurs. For the less energetic Br+ and I+ reactions, dihalogen molecular ions are important products. All these positive ion reactions proceed quite efficiently, i.e., the k are appreciable fractions of k(c) their respective collisional rate coefficients, except for the reactions of Cl2 with the lower energy ions of the spin-orbit triplet of I+, i.e., I+(P-3)1.0), for which k approximately 0.07k(c) this being due to the endothermicities of the reactions. The molecular ion Cl2+ undergoes rapid nondissociative charge transfer with Br2, a process which is, of course, endothermic for the reaction of Br2+ with Cl2 and so no reaction is observed. The less-energetic atomic negative ion reactions proceed-via atom exchange-in which the atomic negative ion of the reactant molecular species and a dihalogen molecule are produced. For those reactions that are exothermic, the k are, within error, equal to (2/3)k(c), implying that they proceed via complexes which separate statistically back to reactants (1/3) and forward to products (2/3). Both the Br- +Cl2 and Cl- +Br2 reactions are somewhat less efficient (i.e., k < 2/3k(c)) a result of the slight endothermicities of the reactions. Of the molecular negative ion reactions, electron transfer is the major process in the Cl2- reaction with Br2, whereas the reaction of Br2- with Cl2 proceeds relatively slowly producing the triatomic ion BrCl2-.