Isotope exchange and structural rearrangements in reactions between size-selected ionic water clusters, H3O~+(H2O)_n and NH_4~+(H2O)_n, and D2O

摘要

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.