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Fig. 5.2 Three distinct low barrier tunneling pathways that rearrange the H bond within a water
dimer. a Acceptor switching that has the lowest barrier of all tunneling pathways, where the
tunneling pathway exchanges the two H in the H-bond acceptor molecule and has been
determined to begin with a flip of the acceptor molecule followed by a rotation of the donor
molecule around its donating O-H bond, and completed by a 180 rotation of the complex about
the O-O bond. b Donor-acceptor interchange in which the roles of the H-bond donor and
acceptor molecules are exchanged. Several possible pathways exist for this interchange, the
lowest barrier path being the geared interchange motion. This pathway proceeds via concerted
rotation of the donor and acceptor molecules and through a trans transition state structure as
shown in Fig.
5.3
. c The bifurcation tunneling motion, where the H bond donor exchanges its
protons, consists of the simultaneous in plane librational motion of the donor with the flip of the
acceptor monomer. This is the highest barrier tunneling pathway
Fig. 5.3 Schematic
illustration of energy diagram
of the donor-acceptor
interchange. The energy
levels split into doublet due to
tunneling
theoretically described the abnormal diffusion behavior [
12
]. They revealed the
donor-acceptor interchange governed by quantum tunneling plays a crucial role in
the diffusion process. Figure
5.4
shows the proposed process of a water dimer on
Pd(111). The theoretical calculations predicted the dimer is bonded to the surface
mainly via the oxygen atom of the donor molecule, and almost barrier-less azimuthal
rotation of the acceptor around the donor (Fig.
5.4
a, b. The diffusion proceeds
through the wagging motion, which brings both water molecules to a similar height
above the surface (Fig.
5.4
b, c), and the subsequent donor-acceptor interchange
(Fig.
5.4
c-e). After the interchange the dimer restores its equilibrium configuration
(Fig.
5.4
e, f). The lowest diffusion barrier of this process is calculated to be 0.26 eV.
This result does not rationalize the rapid diffusion of the dimer compared to monomer
having the barrier of 0.19 eV [
11
]. However the inclusion of H-atom tunneling in the
