Isotope exchange and structural rearrangements in reactions between size-selected ionic waterclusters, H3O+(H2O)n and NH4+(H2O)n, and D2O

Abstract

Hydrogen/deuterium exchange in reactions of H₃O⁺(H₂O)_n and NH₄⁺(H₂O)_n (1 ≤ n ≤ 30) with D₂O has been studied experimentally at center-of-mass collisions energies of ≤0.2 eV. For a given cluster size, the cross-sections for H₃O⁺(H₂O)_n and NH₄⁺(H₂O)_n are similar, indicating a structural resemblance and energetics of binding. For protonated pure water clusters, H₃O⁺(H₂O)_n, reacting with D₂O the main H/D exchange mechanism is found to be proton catalyzed. In addition the H/D scrambling becomes close to statistically randomized for the larger clusters. For NH₄⁺(H₂O)_n clusters reacting with D₂O, the main mechanism is a D₂O/H₂O swap reaction. The lifetimes of H₃O⁺(H₂O)_n clusters have been estimated using RRKM theory and a plateau in lifetime vs. cluster size is found already at n = 10.