Direct visualization of concerted proton tunnelling in a water nanocluster

Abstract

Proton transfer through hydrogen bonds plays a fundamental role in many physical, chemical and biological processes 1-5. Proton dynamics is susceptible to quantum tunnelling, which typically involves many hydrogen bonds simultaneously, leading to correlated many-body tunnelling 6-9. In contrast to the well-studied incoherent single-particle tunnelling, our understanding of many-body tunnelling is still in its infancy. Here we report the real-space observation of concerted proton tunnelling in a cyclic water tetramer using a cryogenic scanning tunnelling microscope. This is achieved by monitoring the reversible interconversion of the hydrogen-bonding chirality of the water tetramer with a chlorine-terminated scanning tunnelling microscope tip. We found that the presence of the Cl anion at the tip apex may either enhance or suppress the concerted tunnelling process, depending on the details of the coupling symmetry between the Cl ion and the protons. Our work opens up the possibility of controlling the quantum states of protons with atomic-scale precision. Many-body correlated tunnelling of protons is extremely sensitive to coupling with the atomic-scale environment owing to the demanding phase coherence between the protons. Spectroscopic techniques are not able to provide such local information because of the limitation of spatial resolution 5-7,10,11. Recently, the scanning tunnelling microscope (STM) has proved to be ideal for probing the intramolecular and intermolecular proton dynamics at the singlemolecule level 12-16. However, most of these studies are focused on classical over-barrier proton hopping, whereas through-barrier quantum tunnelling of protons has yet to be explored. Furthermore, tracking proton motion along hydrogen bonds requires the ability to resolve the hydrogen-bonding directionality 17. Although it was recently demonstrated that non-contact atomic force microscopy (NC-AFM; ref. 18) and state-of-the-art STM-based techniques 19 are able to visualize individual hydrogen bonds with unprecedented resolution, the asymmetry of the hydrogen bond and the extent of quantum delocalization of the shared proton has not been directly distinguishable. Here we show the possibility of directly visualizing the concerted tunnelling of four protons in an individual hydrogen-bonded water tetramer adsorbed on a Au-supported NaCl(001) film. This is achieved by monitoring the reversible interconversion of the hydrogen-bonding directionality and associated chirality of the tetramer using a submolecular orbital-imaging technique 20. To investigate the effect of the atomic-scale environment on the tunnelling process in a well-controlled manner, we functionalize the STM tip apex with a single chlorine anion and then tune the Cl-proton electric coupling in three dimensions with picometre

How Does Water Wet a Surface?

“…22,23,40 On Cu(110), the transfer of H atoms along hydrogen bonded chains was controlled and triggered by excitation of molecular vibrations induced by inelastic tunneling electrons. 23 These results confirm that H atom transfer along hydrogen bonds is possible also on metal surfaces despite the strong binding of the water.…”

Force-induced tautomerization in a single molecule

“…The trans → cis barrier essentially vanishes at Δz ≤ −0.30 Å and the cis → trans barrier is reduced to only 66 and 35 meV at Δz = −0.30 and −0.55 Å, respectively. Thus, at these short distances a small barrier may remain for the first cis→trans conversion, but the reaction could proceed via quantum tunnelling 40,41 .…”

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.…”