Hydrogen bond breakthrough
Image: A hydrogen bond forms between a propellane (lower molecule) and the carbon monoxide functionalized tip of an AFM.[ University of Basel]
In a world first, an international team of researchers has directly detected hydrogen bonds in a single molecule, using atomic force microscopy.
Hydrogen is the smallest, most abundant atom, but while myriad methods have been applied to its analysis, direct observation of the atoms in a single molecule has remained largely unexplored, until now.
Professor Ernst Meyer from the Department of Physics, University of Basel, and colleagues used high resolution AFM with a carbon monoxide functionalised tip to resolve the outermost hydrogen atoms of individual propellane-based molecules.
(A to F) Series of STM topographies (A, C, and E) of trinaphtho[3.3.3]propellane deposited on the Ag(111) surface with increasing coverage and corresponding AFM images (B, D, and F). As the coverage of TNP increases, the ratio of the upright (red arrows) and side-lying (yellow arrow) TNP becomes larger. (G) STM topography of upright trifluorantheno[3.3.3]propellane and (H) corresponding AFM image. [Kawai et al. Science Advances 12 May 2017]
Experiments took place using an Omicron STM/AFM, and observations were based on the weak C═O⋅⋅⋅H–C intermolecular interaction which takes place just before the onset of Pauli repulsion.
The researchers selected propellane as its configuration resembles that of a propeller, with the molecules arranging on a surface in such a way that two hydrogen atoms always point upwards.
Crucially, if the functionalised tip of the AFM is brought close enough to these hydrogen atoms, hydrogen bonds are formed that can then be examined.
According to Meyer: "This very weak interaction... is responsible for the spatially localized contrast of the hydrogen atom and was used as a marker to identify the adsorption geometry of the three-dimensional hydrocarbon."
The measured forces and distances between the oxygen atoms at the AFM tip and the propellane’s hydrogen atoms corresponded very well to density functional theory calculations.
With this study, the researchers have opened up new ways to identify 3D molecules such as nucleic acids or polymers via observation of hydrogen atoms.
"Potentially, this technique can be expanded to identify more complex large molecules such as DNAs and polymers," he adds. "We used a linear O⋅⋅⋅H–C system... which in principle allows us to investigate any kind of intermolecular interactions in a quantitative manner at the atomic scale."
Research is published in Science Advances.