Direct Observation of Hydrogen-Bond Exchange within a Single Water Dimer

The dynamics of water dimers was investigated at the single-molecule level by using a scanning tunneling microscope. The two molecules in a water dimer, bound on a Cu(110) surface at 6 K, were observed to exchange their roles as hydrogen-bond donor and acceptor via hydrogen-bond rearrangement. The interchange rate is $\sim 60$ times higher for $({\mathrm{H}}_2\mathrm{O}{)}_2$ than for $({\mathrm{D}}_2\mathrm{O}{)}_2$, suggesting that quantum tunneling is involved in the process. The interchange rate is enhanced upon excitation of the intermolecular mode that correlates with the reaction coordinate.

Figure 1

(a) Constant current image of ${\mathrm{H}}_2\mathrm{O}$ monomers. The image was acquired at 0.5 nA tunneling current and 24 mV sample bias. (b) An image after the area was scanned at 0.5 nA and 54 mV. This image was acquired at 0.5 nA and 24 mV. The 54 mV scan induced the monomers to migrate and form a dimer (fluctuating image). The apparent heights of the monomer and dimer are 60 and 80 pm, respectively. Each image size is $4\times 16{\mathrm{nm}}^2$.

Figure 2

(a) Constant current image simultaneously acquired for ${\mathrm{H}}_2\mathrm{O}$ and ${\mathrm{D}}_2\mathrm{O}$ dimers ($5.5\times 5.5{\mathrm{nm}}^2$). (b) An enlarged image of the ${\mathrm{D}}_2\mathrm{O}$ dimer ($2.4\times 2.0{\mathrm{nm}}^2$). The asymmetric egg shape reflects the donor-acceptor configuration of the water dimer along $[1\overline{1}0]$. The grid lines represent the lattice of Cu(110) that is depicted through nearby monomers centered on top of Cu atoms. (c) The ${\mathrm{D}}_2\mathrm{O}$ dimer flipped between the two orientations as the tip was scanned horizontally across it. (d) The counterpart of (b) in the same area. All images were acquired at 24 mV sample bias. Tunneling current was 0.5 nA for (a) and 0.3 nA for (b)–(d).

Figure 3

(a) The calculated structure of water dimer on Cu(110) (side view). (b) The same as in (a) from top view. The Cu-O distances for the donor and acceptor molecules are 0.212 and 0.289 nm, respectively. The intermolecular O-O distance is 0.272 nm. (c) The calculated STM image corresponding to (b). The grid lines represent the lattice of Cu(110).

Figure 4

(a) Typical tunneling current recorded with the tip fixed over the ${\mathrm{H}}_2\mathrm{O}$ dimer. The tip was slightly displaced from the center of the dimer along $[1\overline{1}0]$. The sample bias during the measurement was 24 mV. The current jumps between the two states at the moments of interchange. (b) Distribution of the times for the dimer to stay at the two states. The tip height was varied so that the tunneling current was 0.06, 0.25, and 0.7 nA for the high-current state. The latter two data are displaced vertically for clarity. (c) Voltage dependence of the interchange rate for ${\mathrm{H}}_2\mathrm{O}$ dimer (open circles) and ${\mathrm{D}}_2\mathrm{O}$ dimer (open triangles). The tip height was adjusted to give $0.5\pm 0.1\mathrm{nA}$ for the high-current state whose distribution was used to obtain the rate. The arrows indicate the threshold voltages at which the rate starts to increase. The thresholds were determined to be $45\pm 1$ and $41\pm 1\mathrm{mV}$ for $({\mathrm{H}}_2\mathrm{O}{)}_2$ and $({\mathrm{D}}_2\mathrm{O}{)}_2$, respectively. (d) Current dependence of the interchange rate at 24 mV (open squares) and 54 mV (open circles) for $({\mathrm{H}}_2\mathrm{O}{)}_2$ in a logarithmic scale. The slope for the latter is unity. (e) A normal mode for $({\mathrm{H}}_2\mathrm{O}{)}_2$ that couples with the interchange reaction. This mode involves the motions of donor-substrate stretch and acceptor rotation. (f) A schematic diagram of the potential energy along the interchange reaction pathway. The transition state is of $C_2$ symmetry with the Cu-O distances of 0.226 nm. In addition to the intrinsic tunneling between the ground states (blue or dark gray double-ended arrow), the interchange is mediated by the vibrational excitation (red or light gray arrows). The potential barrier is $23\mathrm{kJ}/\mathrm{mol}$ (0.24 eV) while the vertical scale is not realistic.