Direct Observation of Hydrogen-Atom Relay Reactions Using Low-temperature Scanning Tunneling Microscopy

Abstract

Hydrogen-transfer reactions are involved in a wide range of physical, chemical, and biological processes. Owing to its great importance, a vast number of experimental and theoretical studies have been devoted to elucidate the microscopic mechanism of a hydrogen transfer. The hydrogen bond serves as a pathway of the hydrogen transfer and the hydrogen-bonding geometry/strength are directly associated with the potential energy surface of the transfer. It has been proposed that the hydrogen transfer in condensed phases occurs via sequential hydrogen-bonding rearrangements involving multiple breaking and reforming of the covalent and hydrogen bonds, so-called Grotthuss mechanism. Although this concept is widely accepted, it remains a challenging task to directly examine such a transfer process at the single-molecule level. Here we describe the direct observation of the sequential hydrogen-atom transfer in artificial one-dimensional water–hydroxyl chain complexes using a low-temperature scanning tunneling microscope (STM). The water–hydroxyl complexes were assembled on a Cu(110) surface at 6K from single water molecules by STM-induced dissociation and manipulation. The hydrogen relay from the water molecule at one end to hydroxyl group at another end within the one-dimensional complexes is directly visualized. It is found that the relay reaction is triggered by vibrational excitation through inelastic scattering of tunneling electrons. The structure and the transfer dynamics are also investigated by density functional theory calculations, revealing the vibrational modes coupling to the transfer reaction and the reaction pathway.