Symmetric hydrogen bond in a water-hydroxyl complex on Cu(110)

Water-hydroxyl complexes were produced on Cu(110) and characterized by a scanning tunneling microscope (STM) and first-principles calculations. A water molecule was brought to a fixed hydroxyl (OH) group in a controlled manner with the STM, and two kinds of hydrogen-bonded complexes were produced selectively. A side-on complex, in which a water molecule is bonded to an OH group along the atomic row, is metastable with relatively weak hydrogen bond (0.13 eV). On the other hand, a bridge complex, in which a water molecule is bonded to an OH group across the atomic trough, is most stable and characterized by the strong hydrogen bond (0.44 eV) and the short distance between oxygen atoms (2.5 angstrom). The distance is in the range of the low-barrier hydrogen bond, and we show that a symmetric hydrogen bond (HO-H-OH) is formed in the bridge complex, wherein zero-point nuclear motion plays a crucial role.