Termolecular Association Reactions of Mg, Ca, and Ba Ions

Abstract

We have measured termolecular association rate constants of Mg⁺⁺, Ca⁺⁺, and Ba⁺⁺ with a variety of neutrals (Ar, O₂, N₂, CO, CO₂, N₂O, and H₂O). Most neutrals associated to form all species ${\mathrm{M}}^{\mathrm{++}}\mathrm{·(}\mathrm{X}{)}_n$ , where n=1, ···, 6. The association rate constants of double ions are 100–500 times larger than those of single ions. There are smooth changes in rate constants for any neutral as a function of ionic radius, and a definite ordering exists between neutrals. We can understand the general features of such data and some unusual rate constants with one simple hypothesis (which is based on a kinetic model). We hypothesize that for similar ions and neutral species the rate constants for the termolecular association of neutral molecules are correlated with the ion‐molecule bond strengths. With this hypothesis and our data, predictions can be made for termolecular rate constants of other double ions. Our hypothesis allows inference of a kinetic effect in termolecular association reactions; the low frequency vibrational modes of CO₂ increase the rate of association to ions (compared to H₂O). Electrostatic bond energy calculations show the importance of the molecular hyperpolarizability in determining both bond energies (and unusual rate constants) and the changes in bond energies for successive additions of neutrals to the cluster. We also report a few termolecular association rate constants for Ca⁺ and Ba⁺, equilibrium constants for addition to some large clusters, and observations of some additional bimolecular reactions.