Cluster size dependence of the H/D exchange reaction rate constant for protonated water clusters H+(H2O)n (n=2–11)

The collisions of size- and energy-selected H+(H2O)n (n=2–11) with D2O have been investigated using guided-ion beam mass spectrometry. The H/D exchange reaction was observed by incorporation of D2O into H+(H2O)n. The measured rate constants of this reaction are found to depend strongly on the cluster size; the rate constants drastically decrease up to n=6, and then level off for 6⩽n⩽11. This size dependence is discussed in terms of the structure of protonated water clusters: the core H3O+ and H5O2+ ions, so-called Eigen and Zundel cations respectively, are completely surrounded by the first water layer and cause the rate constants for 6⩽n⩽11 essentially insensitive to n.