First-Time View of a Hydrogen Bond
The relative strength of hydrogen’s attraction to oxygen, for example, gives water many of its distinctive and important qualities such as a high boiling point, cohesiveness, and a higher density as a liquid than as a solid. Chemists have long known what a hydrogen bond should look like, but for the first time researchers have produced a detailed image of this molecular bond using a technique called atomic force microscopy.
In the latest study to visualize molecules, Xiaohui Qiu and colleagues at the National Centre for Nanoscience and Technology, China, went one step further. They used the same non-contact AFM as Fischer, but instead of looking for covalent bonds they tweaked it to look for weaker interactions. AFM can be performed in two modes. In contact mode, the tip of an AFM made from silicon or silicon nitride drags on top of a surface. The deflection caused in the tip by the repulsive force of the surface is then processed to create an image of the surface. In the non-contact mode, the cantilever containing the tip is made to oscillate at a resonance frequency just above the surface that is to be imaged. The weak van der Waals forces of the surface decrease the resonance frequency of the cantilever. This change in frequency can then be processed to reveal the image with atomic precision.
Hydrogen bonds are fundamental to the most important molecules in nature. They are responsible for holding the two strands of the double helix of DNA together and many enzymes catalyze reactions by making use of them. These intermolecular bonds can form when a hydrogen that is bonded to a highly electronegative atom interacts with another negatively charged atom. Despite their ubiquity, Qiu says that the ‘nature of a hydrogen bond is still debated’. It has long been considered an electrostatic interaction, but recently it has been suggested that it has chemical bonding characteristics as evidenced by x-ray diffraction experiments.
The results only confirm that AFM can be used to probe the nature of hydrogen bonds. They do not yet take the debate forward on the nature of the bond. ‘The direct observation of hydrogen bonds is consistent with the concept we learned in high school: an electropositive hydrogen atom bridging between two electronegative species X and Y in the designated form of X–H···Y,’ Qiu says. ‘The problem now is to get a deeper understanding of what causes the contrast at the positions of the hydrogen bonds,’ Gross says.