Elucidation of mechanisms of ion-molecule reactions by ICR

The mechanisms of some positive and negative ion-molecule reactions have been determined by a quantitative ion cyclotron double-resonance technique with isotopically labelled reactants. Four types of mechanism were elucidated: charge transfer, ion transfer, atom abstraction and complex formation. Thus, charge transfer is shown to be the only major reaction channel in the systems CO⁺ + CO₂, SO^- + SO₂, and O^- + NO₂, while in the system Cl₂^- + Cl₂ CO ion transfer is the main mechanism, and complex formation with oxygen-atom scrambling is the major channel in O^- + N₂O. The experimental results also point out cases where the reaction proceeds by more than one mechanism as, for example, in the reaction system H₂O⁺ + H₂O.