Chemistry Reference
In-Depth Information
Chapter 5
Water Dimer: Direct Observation
of Hydrogen-Bond Exchange
Abstract I describe the direct observation of H-bond exchange reaction within a
water dimer isolated on a Cu(110) surface in this chapter. A water dimer consisted
of H-bond donor and acceptor molecule can be produced by the association of
water monomers. The STM image of a dimer is characterized by an incessant
fluctuation between two states. Combined with DFT calculations it is found that
the fluctuation corresponds to the H-bond donor-acceptor interchange where the
role of each molecule is exchanged within a dimer. To elucidate the mechanism
the interchange rate is investigated using the time-resolved measurement of STM.
The voltage and current dependence of the rate unveil the interchange is not
induced by STM, thus inherent in a dimer, at low bias voltages (V
s
\ 40 mV).
Furthermore, it is found that the rate of (H
2
O)
2
is *60 times larger than that of
(D
2
O)
2
in the low voltage region, indicating that the interchange process includes
quantum tunneling. The voltage dependence also reveals the interchange rate is
increased by the excitation of the intermolecular vibration mode of a dimer
through the vibrationally assisted tunneling process.
Keywords Water dimer
Hydrogen-bond rearrangements
Quantum tunneling
Vibrationally assisted tunneling
5.1 Introduction
Water dimer has been an enthusiastic research object as a prototype of much more
complex H-bonding systems and called ''a theoretical Guinea pig''. A dimer is the
smallest water cluster composed of H-bond donor and acceptor molecules
(Fig.
5.1
) and regarded as the simplest model of H-bond dynamics. The measured
distance between molecules was 2.98 Å [
1
-
4
] that is significantly longer than that
in both liquid water and regular ice (about 2.85 and 2.74 Å, respectively). The
