Rate estimates for collisions of ionic clusters with neutral reactant molecules

Measured reaction rate constants of ionic clusters often exceed the collision rate calculated with the widely used Average Dipole Orientation (ADO) theory. Accurate trajectory calculations, however, are in most cases not possible since detailed information on the cluster geometry and the interaction potential with the neutral is not readily available. Two simple and universally applicable models to estimate these collision rates are presented here. In both models, the cluster and the neutral reaction partner are treated as hard spheres, and the charge is treated as a point charge. The attraction between the point charge and the neutral molecule is described with the interaction potential from ADO theory. For the Hard Sphere Average dipole orientation (HSA) collision rate, the charge is located in the center of the cluster, while in the Surface Charge Capture (SCC) model, the charge is drawn to the cluster surface by the attractive interaction with the neutral collision partner. Both models lead to significantly increased collision rate constants, in comparison with Langevin or ADO capture rates and purely geometric hard-sphere collisions. For weakly polarizable clusters like (H2O)n−, the newly derived HSA rate constants lie close to experimentally measured rate constants. For the reactions of metal clusters with CO, the SCC model is in surprisingly good agreement with experiment. Although both models are far from being exact theories, they seem valuable for the interpretation of gas phase reactivity experiments of ionic clusters.