Turns out this molecular structure isn’t ‘impossible’

Chemists are offering the first definitive evidence for a new molecular structure: a “supramolecule” with two negatively charged ions.

The existence of a chemical bond between two negatively charged molecules of bisulfate, or HSO4, was considered impossible since it appears to defy a nearly 250-year-old chemical law, called Coulomb’s law, which has recently come under new scrutiny.

“An anion-anion dimerization of bisulfate goes against simple expectations of Coulomb’s law,” says Amar Flood, a chemistry professor at Indiana University and senior author of the study in the journal Angewandte Chemie International Edition. “But the structural evidence we present in this paper shows two hydroxy anions can in fact be chemically bonded. We believe the long-range repulsions between these anions are offset by short-range attractions.”

The chemistry of supramolecules

In molecular chemistry, two monomer molecules connected by a strong covalent bond are called a “dimer.” (A polymer is a chain of many monomers.) In supramolecular chemistry, the dimers are connected by many weak non-covalent bonds. A negatively charged particle is an anion.

A key part of Coulomb’s law is the idea that two molecules with the same charge create a repellent force that prevents chemical bonding—like two magnets with the same end put into close contact. But recently, experts have begun to argue that negatively charged molecules with hydrogen atoms, such as a bisulfate—composed of hydrogen, sulfur, and oxygen—can also form viable chemical bonds.

“I expect this will be the start of something new and important in the field.”

“Although supramolecular chemistry started out as an effort to create new molecular hosts that hold on to complementary molecular guests through non-covalent bonds, the field has recently branched out to explore non-covalent interactions between the guests in order to create new ‘chemical species,’” says Elisabeth Fatila, a postdoctoral researcher in Flood’s lab and the study’s first author.

The negatively charged bisulfate dimer in the study employs a self-complementary, anti-electrostatic hydrogen bond.

The molecule’s existence is made possible through encapsulation inside a pair of cyanostar macrocycles, a molecule previously developed by Flood’s lab. Fatila and colleagues were trying to bind a single bisulfate molecule inside the cyanostar; the presence of two negatively charged bisulfate ions was a surprise.

“This paper is inspirational because it may launch a new approach to supramolecular ion recognition,” says Jonathan Sessler, a professor of chemistry at the University of Texas at Austin who was not involved in the study. “I expect this will be the start of something new and important in the field.”

Reduce nuclear and agricultural waste

The ability to produce a negatively charged bisulfate dimer might also advance the search for chemical solutions to several environmental challenges. Due to their ion-extraction properties, the molecules could potentially be used to remove sulfate ions from the process used to transform nuclear waste into storable solids—a method called vitrification, which is harmed by these ions—as well as to extract harmful phosphate ions from the environment.

“The eutrophication of lakes is just one example of the serious threat to the environment caused by the runoff of phosphates from fertilizers,” Flood says, referring to uncontrolled plant growth that results from excess phosphate nutrients running into lakes and ocean.

When these chemicals get into the water supply as runoff from fertilizer—produced by dairy farms and used to increase crop yields—they can trigger massive algae blooms that poison water supplies and kill fish in large numbers.

The National Science Foundation supported the work.

Source: Indiana University

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