Chemistry Reference
In-Depth Information
Fig. 5.1 The equilibrium structure of water dimer determined by calculations. The H bond
deviates 2.3 from linearity, the O-O distance is 2.952 Å, and the bond strength is 3.40 kcal/mol
shortening of the R(O
O) distance in condensed phase represents the existence of
a stronger H-bonded interaction and the cooperative nature of H bond.
The unambiguous insights into the structure and dynamics of a gas phase dimer
were provided by the high-resolution vibration-rotation spectroscopy [
1
-
8
]. The
pioneering work by Dyke and co-workers revealed that the H-bond exchange occurs
via quantum tunneling in a water dimer [
1
-
4
] and ab initio calculations predicted
three distinct low barrier tunneling pathways as shown in Fig.
5.2
[
9
]. First, in
''acceptor switching'' (Fig.
5.2
a), having the lowest barrier of *20 meV, the
acceptor molecule is inverted where the H bond remains intact during the process.
The second lowest barrier process is ''donor-acceptor interchange'' (Fig.
5.2
b)
where the role of each molecule is exchanged via the concerted rotation and the
barrier is *26 meV. Finally, ''bification'' is the highest barrier process having
*49 meV (Fig.
5.2
c), where the bound and free hydrogen on the donor are
exchanged. Here I focus on the process of donor-acceptor interchange. Figure
5.3
illustrates the potential energy surface of the interchange. The interchange proceeds
through the concerted rotation of each molecule, wherein the transition state has C
2v
symmetry. As described in the Chap. 2, quantum tunneling manifests the splitting of
the energy level and its width represents the tunneling rate. This energy splitting was
directly observed in the vibration-rotation-tunneling spectra. The tunneling rate for
the interchange was estimated to be *10
9
s
-1
for (H
2
O)
2
[
5
]. Moreover, the effect of
vibrational excitation on the interchange tunneling dynamics was also investigated
[
10
] and the tunneling rate was found to be affected drastically by the excitation of the
intermolecular vibration mode that was argued to correlate with the interchange
reaction coordinate.
In general, it is suspicious if we could directly apply the knowledge of H-bond
dynamics of gas-phase species to that of adsorbate systems in which strong envi-
ronmental effects must be included. However, the donor-acceptor interchange in a
gas phase dimer was recently invoked to explain the unique mobility of a water dimer
on a Pd(111) surface. In 2002, Mitsui et al. observed the anomalous diffusion rate of a
water dimer on Pd(111) at 40 K, where the rate of the dimer was much faster than that
of a monomer and other clusters [
11
]. Specifically, the diffusion of a water dimer was
found to be larger than that of monomer and trimer (tetramer) by more than four and
two orders of magnitude, respectively. They claimed the rapid diffusion is associated
with the mismatch between the O-O distance of a dimer (2.95 Å in the gas phase) and
the Pd lattice distance at the surface (2.75 Å). After Mitsui's report Ranea et al.
