Ion-mediated hydrogen-bond rearrangement through tunnelling in the iodide-dihydrate complex

A microscopic picture of hydrogen-bond structure and dynamics in ion hydration shells remains elusive. Small ion-dihydrate molecular complexes are ideal systems with which to investigate the interplay and competition between ion-water and water-water interactions. Here, state-of-the-art quantum dynamics simulations provide evidence for tunnelling in hydrogen-bond rearrangements in the iodide-dihydrate complex and show that it can be controlled through isotopic substitutions. We find that the iodide ion weakens the neighbouring water-water hydrogen bond, leading to faster water reorientation than in the analogous water trimer. These faster dynamics, which are apparently at odds with the slowdown observed in the first hydration shell of iodide in solution, can be traced back to the presence of a free OH bond in the iodide-dihydrate complex, which effectively triggers the overall structural rearrangements within it. Besides providing indirect support for cooperative hydrogen-bond dynamics in iodide solutions, the analysis presented here suggests that iodide ions may accelerate hydrogen-bond rearrangements at aqueous interfaces, where neighbouring water molecules can be undercoordinated.