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A Breakthrough in Imaging Chemical Bonds using Atomic Force Microscopy by Chinese Researchers

On September 26, Science Express published online a report by the researchers from the National Center for Nanoscience and Technology of China (NCNST) and Renmin University that they have successfully obtained the real-space images of hydrogen bonds and coordination bonds formed between molecules using a technique namedatomicforce microscopy (AFM). The result promises significant advances in the ability to investigate intermolecular interactions at the single molecular level. This research work was partially supported by NSFC grants (21173058, 21203038, 11274308, and 11004244).

As one of the most important intermolecular interactions, hydrogen bonding is ubiquitous in nature. Although they are much weaker than the covalent or ionic bonds, hydrogen bonds play major roles in explaining the physical properties of substances in condensed phases. For example, these delicate interactions determine why water exists as liquid at room temperature, and lay the fundamental concept for a variety of biological recognition processes, including the formation of helix structure and protein crystallization. For a long time, the hydrogen bond has served as a representation of a non-covalent bond and is believed to be originated from the electrostatic interaction. However, recent evidence suggested its partly covalent character, i.e. there is a weak sharing of electron pairs between the proton and the two electronegative atoms in the configuration of X-H...Y. In 2011, International Union of Pure and Applied Chemistry (IUPAC) recommended a new definition of hydrogen bond, yet the nature of this bond is intriguing. Experimental studies on hydrogen bonding have been mainly conducted through x-ray and electron diffractions, as well as infrared, Raman, nuclear magnetic resonance. Nevertheless, the detailed information on the bond configurations has to be derived in combination with theoretical simulations. At the single-molecule level, state-of-the-art scanning tunneling microscopy (STM) is capable of achieving atomic resolution, whereas the lack of electronic states near the Fermi level makes hydrogen bond elusive in STM observations so far. Given the importance of further understanding of hydrogen bonding, as well as the emergence of rational drug design and functional materials, it is desirable to invent highly sensitive and precise approaches to investigate intermolecular interactions with atomic or sub-molecular resolution.

The team led by Prof. XiaohuiQiu and Assoc. Prof. Zhihai Cheng at NCNST has been committed to construct and develop advanced techniques for nanocharacterization. They strived to upgrade a commercial AFM system that finally achieved world-class performance in mechanical and electronic stability and signal-to-noise ratio. In ultrahigh vacuum and at cryogenic temperature, the team has obtained the first real-space image of hydrogen bonds present between 8-hydroxylquinoline (8-hq) molecules self-assembled on the Cu(111) surface via measurement of the Pauli repulsion between AFM tip and the electrons in the proximity of the chemical bonds. The atom-resolved molecular structures enable a precise determination of the configurations of hydrogen bonding networks, including the bonding sites, orientations, and lengths. In collaboration with theoretician Assoc. Prof. Wei Ji at RenminUniveristy, the team interpreted the image contrast as the electron density contribution from the hybridized electronic state of the hydrogen bond. Furthermore, the team also observed intermolecular coordination between the dehydrogenated 8-hq and Cu adatoms in the AFM characterization.

The direct identification of local bonding configurations as demonstrated in this research work opens up a new approach to interrogate hydrogen bond. It will greatly facilitate detailed investigations of intermolecular interactions in supramolecular and biological systems in the future.

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