The researchers used infrared light to set bifluoride ions vibrating and measured the hydrogen atoms’ response, revealing a series of energy levels at which the hydrogen atoms vibrated. For a typical hydrogen bond, the spacing between those energy levels would decrease as the atom climbed further up the energy ladder. But instead, the researchers found that the spacing increased. This proves that the hydrogen atom was being shared equally instead of being closely bound to one fluorine atom by a covalent bond and more loosely bound by a typical hydrogen bond to the other.
Computer calculations showed that this behavior is dependent on the distance between the two fluorine atoms. As the fluorine atoms move closer to each other, squeezing the hydrogen between them, the normal hydrogen bond becomes stronger, until all three atoms begin sharing electrons as in a covalent bond, forming a single link that the researchers call a hydrogen-mediated chemical bond. The hydrogen-mediated chemical bond can’t be described as either a pure hydrogen bond or a pure covalent bond, the researchers conclude. “It’s really some hybrid of the two,” says chemist Mischa Bonn of the Max Planck Institute for Polymer Research in Mainz, Germany.
Fluorine atoms (illustrated in green) squeeze a hydrogen atom (orange) between them when dissolved in water (red and silver). Researchers used infrared laser light (red lines) to study the chemical bond that formed (branching blue lines), which acts as a hybrid between a hydrogen bond and a covalent bond.
This discovery is something that will change the course of chemistry forever. The new observation has implications for how scientists understand the basic principles of chemistry. “It touches on our fundamental understanding of what a chemical bond is,” Bonn says. The newfound understanding of chemical bonding also raises questions about what qualifies as a molecule. Atoms connected by covalent bonds are considered part of a single molecule, while those connected by hydrogen bonds can remain separate entities. The bonds that are in limbo between the two raise questions. At what point do you go from a bond with two molecules to a bond with one molecule.
Sources:
Conover, Emily. “This Weird Chemical Bond Acts like a Mash-up of Hydrogen and Covalent Bonds.” Science News, 27 Jan. 2021, https://www.sciencenews.org/article/new-weird-hybrid-chemical-bond-hydrogen-covalent.
This is something that might be interesting to someone who has had some chemistry. The notion of a strong hydrogen bond, while not discussed in basic chemistry classes, is not new. The bond between F- and HF has long been known to be quite strong and assumed to have some covalent character. There is a similar strong hydrogen bond between H3O+ and H2O. These kinds of interactions are important in a wide variety of chemical systems including enzymatic catalysis. The detailed potential is new. That is it is the asymmetric double welled potential characterized by vibrational spectroscopy that is new. This is from the results of an impressive femtosecond laser experiment. Your title will interest those with a bit of chemical sophistication. The figures are interesting and appropriate. Figure captions like your figure 3 are useful. For example your figure 2 could use a caption. I assume that the x-axis on these plots represents the D-H distance at fixed D-A distance. These plots might be a bit puzzling for the chemically unsophisticated. I suppose Science News is supposed to be a general interest publication, but it is probably not as widely accessible as other general interest news sources like major newspapers, network television news outlets and so on. On the whole though pretty interesting stuff.
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