Water: now we see it, see more

APR . 16 2016
Peking University, Apr. 15, 2016: The mystery of water mainly arises from the intermolecular hydrogen-bonding interaction. It is well known that hydrogen bonds have a strong classic component coming from electrostatics. However, its quantum component can be exceptionally prominent due to the zero-point motion of light hydrogen nuclei (proton), which is a natural result of the Heisenberg uncertainty principle. Therefore, the assessment of nuclear quantum effects has been a key issue for understanding the structure, dynamics, and macroscopic properties of water. Despite enormous scientific efforts in past decades, it still remains an open question to what extent the quantum motion of the hydrogen nuclei can affect the hydrogen bond.
Now, the teams led by Prof. Jiang Ying and Prof. Wang Enge of International Center for Quantum Materials (ICQM) of Peking University provide a smoking gun for this important question. As published in Science on Apr. 15, 2016 (Science DOI: 10.1126/science.aaf2042), the researchers unravel quantitatively, for the first time, the quantum component of a single hydrogen bond at a water-solid interface, through a combined study using a scanning tunneling microscope (STM) and density functional theory (DFT) calculations.
"The main difficulty of extracting the quantum component of hydrogen bond lies in that the quantum states of hydrogen nuclei are extremely sensitive to the coupling with local environments, leading to significant broadening and averaging effects when conventional spectroscopic or diffraction techniques are used," says Jiang. Therefore, the ability to probe water with single bond precision is crucial.
To this end, the researchers succeeded to push the limit of vibrational spectroscopy of water down to the single-bond level using a novel technique called tip-enhanced inelastic electron tunneling spectroscopy (IETS) based on STM, which combines sub-ångström spatial resolution and single-bond vibrational sensitivity. The signal-to-noise ratios of the tip-enhanced IETS are enhanced by orders of magnitude over the conventional STM-IETS, which was pioneered by Prof. Wilson Ho's group of UC Irvine 18 years ago.
"The conventional IETS signals of water are extraordinarily weak since the frontier orbitals of water are located far away from the Fermi level. The key to defeat this limitation is gating the frontier orbitals of water towards the Fermi level with a chlorine-terminated STM tip to resonantly enhance the electron-vibration coupling," explains Jiang. With such a tip-enhanced IETS, the hydrogen-bonding strength can be determined with unprecedentedly high accuracy from the redshift in the O-H stretching frequency of water.
By conducting isotopic substitution experiments (replacing hydrogen atom with heavier deuterium atom), the researchers could extract the quantum component of the hydrogen bond, which accounts for up to 14% of the bond strength. Surprisingly, the quantum contribution is much greater than the thermal energy contribution, even at room temperature. In-depth investigation combined with ab initio path integral molecular dynamics (PIMD) simulations reveal that the anharmonic quantum fluctuations of hydrogen nuclei weaken the weak hydrogen bonds and strengthen the strong ones. However, this trend can be completely reversed when the hydrogen bond is strongly coupled to the polar atomic sites of the surface.
"This joint experimental and theoretical work yields a cohesive picture for the nuclear quantum effects of hydrogen bonds," adds Wang, "Those findings may completely renovate our understanding of water and provide answers to many weirdness of water from a quantum mechanical view. It would be very interesting to further explore the quantum effects on the cooperativity of correlated H-bonds beyond the single hydrogen bond." 
This work received supports from Ministry of Science and Technology of China, National Natural Science Foundation of China, Ministry of Education of China, National Program for Support of Eminent Professionals, and Collaborative Innovation Center of Quantum Matter, China.

Figure caption: Left is the schematic of STM experimental setup. The hydrogen atoms of water show prominent zero-point motion thanks to the Heisenberg uncertainty principle. Right is the tip-enhanced IETS of a single water molecule, in which stretching, bending and rotational modes are identified. Those vibrational modes can be used as sensitive probes to sense the influence of quantum motion of hydrogen nuclei on the hydrogen bond. (Design: Liang Mingcheng)

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Edited by: Zhang Jiang
Source: School of Physics