Scientists have recently discovered a completely new type of bond and it’s way stronger than it’s any right to be.
The new type of bond shows that the divide between powerful covalent bonds, which bind molecules together & weak hydrogen bonds which form between molecules & may be broken by something as simple as stirring salt into a glass of water, isn’t as clear as chemistry textbooks would suggest.
Ionic bonds link metals & non-metals to make salts. Strong covalent bonds bind together molecules like CO2 & water. Far weaker hydrogen bonds form due to an electrostatic force of attraction between hydrogen & a more charged atom or molecule, as an example, causing water molecules to attract each other & form droplets or crystalline ice. Ionic, covalent & hydrogen bonds are all relatively stable; they have a tendency to last for extended periods of time & have effects are easily observable. But researchers have long known that during a reaction as chemical bonds are forming or breaking, the story is more complicated & involves “intermediate states“ which may exist for little fractions of a second & are harder to observe.
In the new study, researchers managed to stay these intermediate states going for long-enough to make a detailed-examination. What they found was a hydrogen bond with the strength of a covalent bond, binding atoms together-into something resembling a molecule.
To do that, researchers dissolved a hydrogen-fluoride compound in water, and watched how the hydrogen & fluorine atoms interacted. The fluorine atoms were attracted towards the hydrogen atoms due to imbalances of positive & negative charges across their surfaces, the classic structure of a hydrogen-bond. Each atom tended be sandwiched between 2 fluorine atoms. But those sandwiches were bound together with more-strength than typical hydrogen bonds which are easily broken. The hydrogen atoms bounced back & forth between the fluorine atoms, forming bonds as strong as covalent bonds & resembling molecules, which hydrogen bonds should not be able to form. But the mechanism of the new bond was electrostatic, which means it involved the type of differences in positive & negative charge that define hydrogen bonds.
The new bonds had a strength of 45.8 kilocalories per mole (a unit of chemical bonding energy), greater than some covalent bonds. Nitrogen molecules, for instance, are made from 2 nitrogen atoms bound together with a strength of about 40 kcal/mol, consistent with LibreTexts. A chemical bond typically has an energy of about 1–3 kcal/mol, consistent with the book Biochemistry.
They explain their results to a paper published on Thursday (January 7, 2021) in the journal Science. In an accompanying article in Science, Mischa Bonn & Johannes Hunger, researchers at Planck Institute for Polymer Research in Germany, who weren’t involved in the study, wrote that this unusual bond blurs the clear categories of chemistry.
“The existence of a hybrid covalent hydrogen bonded state not only challenges our current understanding of what a chemical bond exactly is, but also offers the chance to better understand chemical reactions.” They wrote, “where ‘intermediate reaction states’ are often invoked but rarely studied directly.”
Similar bonds likely exist in pure water, they wrote, when a hydrogen atom finds itself sandwiched between 2 water molecules. But those bonds are believed to exist but not be as long lived, the researchers wrote. And they have never been conclusively observed.
This study, they wrote, could open the door to a “deeper understanding of strong-bonding” & the intermediate reaction states.