Chemistry Reference
In-Depth Information
Fig. 10.11 Calculated
potential energy surface for
the H-atom transfer reaction
of H
2
O-(OH)
2
on a Cu(110)
surface. The H-atom moves
along [001] direction. The set
of images from initial to final
states, shown as insets, were
determined by the nudged
elastic band method. The H-
transfer to the center OH is
facile (*0.04 eV). The
transition state (0.25 eV)
corresponds to OH-H
2
O-OH
with a broken H bond
H
2
O scissors modes are therefore postulated to couple to the reaction coordinate
for the H-bond cleavage. It is noted that the H-atom transfer is observed below the
bias voltages of 250 mV in the bias voltage dependence (Fig.
10.8
a), where
tunneling electron should not have enough energy to induce the H-atom transfer
via the over-barrier process. Although it is still open question, I propose that the
H-bond cleavage, which mainly involves the rotational motion of a center water
molecule, might here proceed via vibrationally-assisted tunneling [
9
] of an entire
H
2
OorD
2
O molecule through the transition state. The transfer reaction is
therefore completed either via tunneling (at low bias) or over-barrier processes (at
high bias). For longer chains [H
2
O-(OH)
3,4
], the H-atom transfer reaction occurs
in sequence and thus the energy dissipation is more significant, giving rise to lower
reaction yields than for an H
2
O-(OH)
2
. Such energy dissipation plays a crucial
role in determining the product, yield, and pathway in chemical reactions on
surface.
10.3 Summary
H-atom relay reactions were visualized with one-dimensional H-bonded water-
hydroxyl complexes assembled with the single molecule precision using STM. The
H-atom relay reaction is induced by vibrational excitations, where the water
molecule situated at the end of the complex is structurally transferred to another
end via the multiple H-/covalent bond exchange. The voltage, current, and spatial
dependence of the relay rate (yield) unveiled the behind mechanism. In the voltage
dependence, the increase of the relay yield was observed at vibrational energies of
the scissors, shared OH, and free OH stretch modes with the rational isotope shift.
The current dependence revealed the relay is driven by one-electron processes in
the whole voltage region. Furthermore the spatial dependence of the yield showed
the vibrational excitation is localized over a water molecule at the energy of the
