A new study published in the journal Science succeeded for the first time in observing one of these intermediate states, revealing a new type of chemical bond: a hydrogen bond but with the strength of a covalent bond (in principle, the bond hydrogen is about 20 times weaker). To do this, the researchers dissolved a hydrogen-fluoride compound in water and watched how the hydrogen and fluorine atoms interacted using a spectroscopy device that recorded the vibrations of the atoms under infrared light. They found that fluorine atoms were attracted to hydrogen atoms due to an imbalance of positive and negative charges, as in a standard hydrogen bond. In the bifluoride anion solution, each hydrogen atom thus tends to be “sandwiched” between two fluorine atoms.
But the researchers found that these sandwiches bonded together with more force than typical hydrogen bonds. In these short hydrogen bonds, the atoms come together in a molecule-like structure, which is normally characteristic of a covalent bond. But the mechanism of the new bond being of an electrostatic nature, this means that the bond remains of Van der Waals nature.
According to researchers at the Max-Planck Institute (not involved in the study), it is possible that this type of bond exists in water, where a hydrogen ion is sandwiched between two water molecules. These links have never yet been conclusively observed because their lifespan would be much shorter than with fluorine. “Studying these molecular motions and how they can be oriented in a desired direction could find applications in proton transport in biology and in technologies such as fuel cell membranes,” the researchers say.