Chemistry students around the world know about covalent bonds and hydrogen bonds. Now a study has revealed a strange variety of links that act as a hybrid of the two. Its properties raise questions about how chemical bonds are defined, chemists report on Jan. 8.
Hydrogen bonds are usually considered weak electrical attractions rather than true chemical bonds. Covalent bonds, on the other hand, are strong chemical bonds that bind atoms within a molecule and result in electrons sharing between atoms. Now, researchers have reported that an unusually strong variety of hydrogen bonding is in fact a hybrid, as it involves shared electrons, blurring the distinction between hydrogen and covalent bonds.
“Our understanding of the chemical bond, as we teach it, is very black and white,” says chemist Andrei Tokmakoff of the University of Chicago. The new study shows that "there is actually a continuum."
Tokmakoff and colleagues characterized the hybrid bond by observing groups of atoms called bifluoride ions, consisting of a single hydrogen atom interspersed between a pair of fluorine atoms, in water. According to conventional wisdom, the hydrogen atom is attached to one fluorine by a covalent bond and to the other fluorine by a hydrogen bond.
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The researchers used infrared light to establish that bifluoride ions vibrated and measured the response of hydrogen atoms, revealing a series of energy levels at which hydrogen atoms vibrated. For a typical hydrogen bond, the space between those energy levels would decrease as the atom climbed further down the energy ladder. But instead, researchers found that spacing was increasing. This behavior indicated that the hydrogen atom was shared between the two fluorine atoms equally, rather than being tightly attached to a fluorine atom by a covalent bond and more closely linked by a typical hydrogen bond to the other. In that arrangement, “the difference between the covalent bond and [hydrogen] clears and no longer makes sense,” says study co-author Bogdan Dereka, a chemist also at the University of Chicago.
Computer calculations have shown that this behavior depends on the distance between the two fluorine atoms. As fluorine atoms approach each other, squeezing hydrogen together, the normal hydrogen bond becomes stronger, until the three atoms begin to share electrons as in a covalent bond, forming a single bond that researchers call hydrogen-mediated chemical bonding. . For fluorine atoms that are further away, the conventional description still applies, with distinct covalent and hydrogen bonds.
The researchers conclude that the hydrogen-mediated chemical bond cannot be described as a pure hydrogen bond or a pure covalent bond. “It’s really a hybrid of the two,” says chemist Mischa Bonn of the Max Planck Institute for Polymer Research in Mainz, Germany, who co-authored a perspective piece of the study, also published in Science.
Hydrogen bonds are produced in a variety of substances, the most famous being in water. Without hydrogen bonds, water at room temperature would be a gas instead of a liquid. Although most hydrogen bonds in water are weak, strong hydrogen bonds similar to those found in bifluoride ions can form in water that contains excess hydrogen ions. Two water molecules can intercalate a hydrogen ion, creating what is called a Zundel ion, in which the hydrogen ion is shared equally between the two water molecules. The new results echo the behavior of the Zundel ion, says chemist Erik Nibbering of the Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy in Berlin, who co-authored a 2017 article in Science on the Zundel ion. "Everything fits really well."
Strong hydrogen bonds are believed to play a role in the transport of hydrogen ions, a crucial process for a variety of biological mechanisms, including power cells, and technologies such as fuel cells. So, better understanding these links could shed light on a variety of effects.
And 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.
That new understanding of the chemical bond 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. So the links in limbo between the two lead to the question, "when do you go from two molecules to one molecule?" In Tokmakoff.