Where the hydrogen bond ends and the covalent bond begins

Open an introductory chemistry textbook. What is a covalent bond? A covalent bond occurs when atoms share electrons in a molecule. What is a hydrogen bond? A hydrogen bond is an electrostatic attraction between an atom and the positive charge of a hydrogen atom covalently bound to something else. It is weaker than a covalent bond and can be either inter- or intramolecular.

New research suggests this neat differentiation doesn’t tell the full story. Using spectroscopic experiments and computer simulations, Andrei Tokmakoff at the University of Chicago and colleagues show that the line between a hydrogen bond and a covalent bond is blurrier than your textbook suggests (Science 2021, DOI: 10.1126/science.abe1951) .

Hydrogen bonds are responsible for the lattice structure of frozen water, for example, and for joining the two strands of DNA’s double helix. Chemists know that some hydrogen bonds are stronger than others. Tokmakoff, who thinks a lot about hydrogen bonds in his work on the properties of water and aqueous acid solutions, says he has often wondered how strong hydrogen bonds can get. Or, put another way, how close hydrogen bonds can get to becoming covalent.

In part to try to answer that question, postdoctoral researcher Bogdan Dereka used 2-D femtosecond infrared spectroscopy to study a simplified system, the F–H–F^– ion in aqueous solution. In F–H–F^–, a fluorine anion forms a hydrogen bond with the hydrogen in a covalent HF molecule.

The spectra revealed the many states in which this ion exists in solution, which depend in part on the arrangement of water molecules in the ion’s immediate environment. The hydrogen bonds occur in a range of lengths and strengths. A shorter hydrogen bond, for instance, indicates stronger hydrogen bonding and a relatively weaker covalent bond. This was reflected, as expected, in lower spectroscopic frequencies for the H–F bond. But at a certain point along this continuum, the researchers found, the trend reversed, and the strength of the covalent bond increased even as the hydrogen bond shortened. Tokmakoff says that’s the behavior of a hydrogen shared equally between the two fluorines, best described in chemistry textbook terms as a covalent bond.

Computer simulations of the system by Joel M. Bowman of Emory University were key to understanding this behavior, and they suggest the shift from conventional hydrogen bond to something more like a covalent bond happens when the H and the F^– are about 2.4 Å apart.

“The authors demonstrate convincingly that the nature of the bonds changes” from hydrogen bonds to covalent bonds, says bonding expert Gernot Frenking of Philipp University of Marburg, adding that a figure from this new paper showing the transition in vibrational behavior now belongs in physical chemistry textbooks. Other experts agree that this more nuanced picture of hydrogen bonding belongs in textbooks, but also point out that theoretical chemists and experimentalists have already developed models of hydrogen bonding over the past few decades that fit what Tokmakoff and colleagues found. Theoretical chemist Anastassia Alexandrova of the University of California, Los Angeles, called the level of detailed information on the bond "beautiful" and says she hopes the researchers will use the methods to investigate other bonds.

Tokmakoff says he will now think differently about the role hydrogen bonds play in aqueous chemistry. For example, the group’s experiments suggest these covalent-like bonds might appear momentarily as water molecules jostle around each other in solution. And since many think strong hydrogen bonds could play a role in things like hydride transport in proteins or proton transport in fuel cells, understanding the bonds better could perhaps help scientists better engineer these processes.