Chemistry Reference
In-Depth Information
Chapter 9
Water-Hydroxyl Complexes: Direct
Observation of a Symmetric Hydrogen
Bond
Abstract I describe the hydration of a hydroxyl group with a water molecule on a
Cu(110) surface and the characterization of water-hydroxyl complexes in this
chapter. Two different structural isomers are selectively produced depending on
the initial geometry of the reactants before the reaction. These isomers are
employed as a model system to examine the nature of H-bond. A combination of
STM experiments with DFT calculations reveals that one of the isomers forms ''a
low-barrier H bond'' due to the strong interaction between water and hydroxyl,
where the zero-point nuclear motion plays a crucial role to determine the structure.
Keywords Hydration reaction
Water-hydroxyl complex
Low-barrier hydrogen
bond
9.1 Introduction
Nature of shared H atom/proton in H bond is a key interest to understand H/proton-
transfer reactions, which plays important roles in the elementary process of
chemical and biological reactions [
1
-
4
]. It is well-known that proton shows an
anomalously high mobility in liquid water [
5
]. In order to explain this anomaly
Grotthuss proposed the idea of ''structural diffusion'' in 1806 [
6
]. In the Grotthuss
mechanism proton transfer is described by a sequential exchange of H- and
covalent-bonds between water molecules as shown in Fig.
9.1
. This concept has
been refined by invoking thermal hopping, proton tunneling, and solvation effects.
The Zundel and Eigen cations (Fig.
9.2
) were proposed to be plausible hydrated
structures associating with the transfer process. The strength and symmetry of the
H bond in those cations are primary issues. In general, the strength of H bond is
reflected in the distance between two O atoms bridged by H atom [
7
,
8
]. The O-O
distance (d
O-O
) determines the potential energy surface (PES) of a shared H/proton
