Water Dimer: Direct Observation of Hydrogen-Bond Exchange
I describe the direct observation of H-bond exchange reaction within a water dimer isolated on a Cu(110) surface in this chapter. A water dimer consisted of H-bond donor and acceptor molecule can be produced by the association of water monomers. The STM i
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Water Dimer: Direct Observation of Hydrogen-Bond Exchange
Abstract I describe the direct observation of H-bond exchange reaction within a water dimer isolated on a Cu(110) surface in this chapter. A water dimer consisted of H-bond donor and acceptor molecule can be produced by the association of water monomers. The STM image of a dimer is characterized by an incessant fluctuation between two states. Combined with DFT calculations it is found that the fluctuation corresponds to the H-bond donor–acceptor interchange where the role of each molecule is exchanged within a dimer. To elucidate the mechanism the interchange rate is investigated using the time-resolved measurement of STM. The voltage and current dependence of the rate unveil the interchange is not induced by STM, thus inherent in a dimer, at low bias voltages (Vs \ 40 mV). Furthermore, it is found that the rate of (H2O)2 is *60 times larger than that of (D2O)2 in the low voltage region, indicating that the interchange process includes quantum tunneling. The voltage dependence also reveals the interchange rate is increased by the excitation of the intermolecular vibration mode of a dimer through the vibrationally assisted tunneling process.
Keywords Water dimer Hydrogen-bond rearrangements Quantum tunneling Vibrationally assisted tunneling
5.1 Introduction Water dimer has been an enthusiastic research object as a prototype of much more complex H-bonding systems and called ‘‘a theoretical Guinea pig’’. A dimer is the smallest water cluster composed of H-bond donor and acceptor molecules (Fig. 5.1) and regarded as the simplest model of H-bond dynamics. The measured distance between molecules was 2.98 Å [1–4] that is significantly longer than that in both liquid water and regular ice (about 2.85 and 2.74 Å, respectively). The
T. Kumagai, Visualization of Hydrogen-Bond Dynamics, Springer Theses, DOI: 10.1007/978-4-431-54156-1_5, Ó Springer Japan 2012
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5 Water Dimer: Direct Observation of Hydrogen-Bond Exchange
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(OO) 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. 5.2a), 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. 5.2b) where the role of each molecule is exchanged via the concerted rot
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