Water Dimer Diffusion on Pd{111} Assisted by an H-Bond Donor-Acceptor Tunneling Exchange

Abstract

Based on the results of density functional theory calculations, a novel mechanism for the diffusion of water dimers on metal surfaces is proposed, which relies on the ability of H bonds to rearrange through quantum tunneling. The mechanism involves quasifree rotation of the dimer and exchange of H-bond donor and acceptor molecules. At appropriate temperatures, water dimers diffuse more rapidly than water monomers, thus providing a physical explanation for the experimentally measured high diffusivity of water dimers on Pd[111] [Science 297, 1850 (2002)]].

Chemical reactions of water molecules on Ru(0001) induced by selective excitation of vibrational modes

“…This indicates a lower diffusion energy barrier than for monomers which, as we have seen, requires excitation with electrons of energy larger than 445 meV. An easier diffusivity of dimers compared to monomers was also observed on Pd(111) 14 , which Ranea et al 22 explained as due to a cooperative mechanism where the donor-acceptor role is successively exchanged when the two molecules bond metastably to the metal atoms. Interestingly a similar experiment performed on rosette- like rotating dimers on Pt(111) by Motobayashi et al 25 , indicates a higher diffusion barrier than for monomers.…”

“…Although the internal structure of these clusters was not usually resolved in the STM images, the number of constituent molecules in the cluster could be found by disrupting them with voltage pulses. In the case of dimers we could observe the peculiar helicopter-like rotation of the acceptor molecule predicted by Ranea et al 22 In this model one water molecule, the donor molecule, is bound to the surface via de O-lone pair orbital while the other is a H-acceptor molecule weakly interacting with the substrate. In our case, the rotation is enhanced by interaction with the tip at low gap resistance (50 mV, 400 pA), as shown in Figure 5, which moves the molecule to the tip position over Figure 3c.…”

“…The rate was determined from the average time required to initiate the process, as shown in the inset on top of Figure 4. For isolated H 2 O (I), the slope of the log-log plot gives N = 1.1 ± 0.2, than 5,[22][23][24] The fact that double or triple excitation of the scissor mode was not observed to induce water diffusion on Ru under the range of tunnel conditions explored in this work places a lower limit on the decay rate for this mode.…”

The water dimer I: Experimental characterization

“…10 4 times larger than other water clusters) of water dimers was attributed to the strong hydrogen bond between the water molecules and a mismatch between the dimer O O (2.96 ± 0.05Å from gas phase experiments) bond length and the Pd(1 1 1) lattice constant (2.75Å). Later, Ranea et al presented a novel mechanism for the diffusion of water dimers on Pd(1 1 1) surface based on density functional theory [96].…”

Decisive role of nuclear quantum effects on surface mediated water dissociation at finite temperature

“…Moreover, the structural, dynamic, and electronic properties of water are known to be heavily affected by the quantum nature of the nuclei even at room temperature [7][8][9][10][11][12][13][14] . In fact, nuclear quantum effects (NQE) have also been shown, through several experiments and a few theoretical works, to play a crucial role in the behaviour of organic adsorbates on metallic surfaces [15][16][17][18][19][20][21][22][23][24] . It is thus to be expected that both conformational entropy and nuclear quantum contributions impact the physics underlying the processes of water adsorption and dissociation on metallic surfaces.…”