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Water is one of the most studied substances with a variety of experimental and theoretical methods. The hydrogen bond characterizes the unique properties of water, and numerous investigations have been devoted to elucidate the hydrogen-bonding structure and dynamics at the molecular level. Being the lightest element, the static and dynamic properties of hydrogen are susceptible to nuclear quantum effects, e.g., tunneling and zero-point motion, which makes the accurate description of the hydrogen bond elusive. As the archetype of the hydrogen bond in water, the gas-phase water dimer has been intensively examined and the structure and hydrogen-bond rearrangements were revealed by a combination of high-resolution vibration–rotation spectroscopy and quantum chemical calculations. The small water clusters on well-defined surfaces can also serve as a model system of extended water layers, which are related to important surface processes such as electrode reactions, heterogeneous catalysis, and energy conversions. However, the hydrogen-bonding structure and dynamics at water–metal interfaces remains poorly understood at the molecular level. In this article, we describe the structure and the hydrogen-bond exchange reaction of the water dimer isolated on a Cu(110) surface at 6 K using a scanning tunneling microscope (STM). The water dimer, consisting of a hydrogen-bond donor and acceptor molecule, was produced from single water molecules by STM manipulation. It was found that the dimer shows a spontaneous donor–acceptor interchange motion even at 5 K, and its rate exhibits a large kinetic isotope effect. We quantified the interchange rate of 60 and 1 s−1 for (H2O)2 and (D2O)2. Density functional theory calculations revealed the structure at the ground and transition states of the interchange motion with the reaction barrier of 0.24 eV that cannot be overcome at 5 K. These results clearly indicate that the interchange is dominated by quantum tunneling. Furthermore, it was found that vibrational excitation of the dimer enhances the reaction rate via an assisted tunneling process.
