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Fig. 9.9 STM simulation for
the asymmetric configuration.
The shared H makes a
covalent bond with the one O
atom in (a). b STM
simulation image is
characterized by asymmetric
character, in which the
maximum protrusion can be
observed over the water unit
molecule isolated on Cu(110) was calculated to be 0.34 eV, a water molecule in
the complex is stabilized totally by as large as 0.78 eV with respect to that in gas
phase. A [001]-complex is assigned to this structure. This structure is, however,
incompatible with the STM appearance of C
2v
. Indeed, the simulated STM image
for this structure (Fig.
9.9
b) has a maximum protrusion over the water molecule.
Thus the asymmetric character is clearly inconsistent with the experimental result.
9.2.3 Formation of a Symmetric Hydrogen Bond
To solve the contradiction between the experiment and calculations for a [001]-
complex, I postulated that the shared H in a [001]-complex is located at the center
between two O atoms due to the formation of a low-barrier H bond, yielding a
symmetric configuration as shown in Figs.
9.10
a. It is well known that the
hydroxide ion (OH
-
) is an excellent H bond acceptor and forms a strong
H bond with water molecule [
24
,
25
]. In fact, the calculations predict that a [001]-
complex is characterized by the short d
O-O
(2.5 Å) and the strong H bond
(0.44 eV). These results imply that the formation of a low-barrier H bond is
plausible in a [001]-complex. The STM image simulated for this symmetric
structure (Fig.
9.10
b) shows a symmetric character, being very consistent with the
experiment. Since the ZPE of nuclear is not included in the present calculations,
where nuclear is treated as a classical particle, the structure of Fig.
9.8
b represents
classically favorable structure. Thus the inclusion of the ZPE possibly gives a
different result.
To examine the speculation that the symmetric state (Figs.
9.10
a) is preferred to
the asymmetric one (Fig.
9.8
b), we first calculated the adiabatic potential energy
surface of the shared H when it is transferred between two O atoms. The adiabatic
potential energy is shown in Fig.
9.11
(circles). At each point along the path, the
positions of O atoms, residue H atoms and the topmost two Cu layers were
