Chemistry Reference
In-Depth Information
Fig. 9.3 Schematic energy diagram of a shared H/proton. a In a normal H bond the shared H/
proton is in a double well potential and the ZPE is well-below the barrier height E
b
. The wave
packet is localized in the well and the H-transfer is described by over-barrier (classical) or
tunneling (quantum) process. b The compression of the d
O-O
causes the significant reduction of
the E
b
and the ZPE eventually exceeds the E
b
. The H-transfer via tunneling is negligible and the
simple transition-state theory no longer works
and yield a pear-shaped protrusion (label this product as [110]-complex).
The reaction occurs spontaneously when the reactants sufficiently come close to
each other. A product complex is never dissociated into the parent water and
hydroxyl under low bias conditions, suggesting it is sufficiently stable compared to
an isolated water and hydroxyl. A [110]-complex can be flipped between two
orientations with a voltage pulse of STM (Fig.
9.4
d-e). The thresholds of the
flipping are roughly estimated to be * 70 and *170 mV for H- and D-com-
plexes, respectively. As a higher bias voltage (*200 mV) is applied, a [110]-
complex dissociates into a water molecule and hydroxyl group. Thus, we can
control the formation and dissociation of H bond in reversible fashion at the single
molecule precision. Figures
9.5
a-d show the sequential images of the formation
and dissociation of the complex. The flip motion and dissociation can be directly
monitored by recording tunneling current during a voltage pulse with the feedback
turned off. Figure
9.5
(e) shows a typical result measured over an H
2
O-OH [110]-
complex at V
s
= 179 mV. The flipping gives relatively wide feature in the high
current state and the abrupt drops in the current correspond to the moment of the
dissociation. In Fig.
9.5
e, the dissociation occurs twice and the complex was
reformed after the first dissociation and finally dissociated into a water and
hydroxyl. The dissociation rate was estimated by repeating the measurement and
the threshold of the dissociation was determined to be *180 mV, indicating the
water scissor mode is associated with the dissociation process. For a D
2
O-OD
[110]-complex, the threshold was *220 mV, suggesting the overtone excitation
of the D
2
O scissors mode.
