Complex formation and decay in ion-molecule reactions: mode-selective scattering as a dynamical probe

The potential energy surfaces for ion-molecule reactions typically have sev eral minima that lie lower in energy than the asymptotes for the reactant a nd product channels. Such complexes can be covalently bound, hydrogen bonde d or electrostatically bound. The nature of these complexes can have dramat ic effects on the outcome of the reaction. We review three experimental stu dies that show three different cases of such influence. The reaction of ace tylene cation with methane shows channels that proceed via a long-lived, co valently bound complex, and one channel that proceeds by a short-lived, wea kly bound encounter complex. Specific reactant vibrations strongly influenc e both total reactivity and branching between the product channels, and the effect is related to the mode-specific coupling of vibration to the reacti on coordinate. The reaction of phenol cation with ammonia is a case involvi ng both electrostatically bound and hydrogen-bonded complexes, with ring-co ordination complexes acting as precursors to the formation of the hydrogen- bonded complexes where reaction can occur. Again, vibration has a significa nt effect, related to the isomerization between coordination and hydrogen-b onded geometries. Finally, the reaction of ammonia cation with methanol inv olves competition between formation of different hydrogen-bonded intermedia tes, and the complex that forms largely determines the outcome of the react ion.