“The Molecule ‘Carbonic Acid’ Seen for the First Time”: A Science Appetizer Based on the Latest Research Published in Inorganics

The article entitled "The Crystal Structure of Carbonic Acid", authored by our Section Editor-in-Chief Prof. Dr. Richard Dronskowski, among others, was published in the Inorganics (ISSN: 2304-6740) Special Issue "Inorganics: 10th Anniversary" and has been discussed in various media outlets. Prof. Dr. Richard Dronskowski has written the following science appetizer based on his latest research:

Why is it that a glass of cool champagne, an early afternoon gin and tonic, and a freshly zapped beer are so desirable to us? It is, in addition to the ravishing taste and a little alcohol, of course, the carbonic acid that makes these drinks sparkle. However, what exactly is carbonic acid, the concentration of which is chemically specified on every soda bottle? Carbonic acid is a mysterious molecule whose existence was essentially unclear until the year 2022, although alchemists must have already touched upon it centuries ago. The so-called “molecular formula” of the compound is amazingly brief: chemists speak of “H₂CO₃”, that is, the simplest thinkable compound of equally simple compounds, pure water, H₂O, and pure carbon dioxide, CO₂, because H₂CO₃ = H₂O + CO₂. It could not be simpler and, in this simplicity, there is very difficult chemistry.

We believe carbonic acid can be recognized in some ways, not just in the drinks mentioned above; in fact, we only perceive its gaseous decay product (the bubbly gas CO₂). So, we never see carbonic acid directly, but only after it has already decayed. Does the substance really exist, or has it only sprung from chemical imagination? Even to this day, most standard textbooks of chemistry claim that carbonic acid does not exist, at least the molecular structure with one carbon atom, three oxygen atoms and two hydrogen atoms has never been observed, astonishingly enough. This challenge was taken up, and after a considerable amount of work, Prof. Dronskowski’s team at RWTH Aachen University in Aachen, Germany, together with colleagues at the Hoffmann institute in Shenzhen, China, and the Technical University of Munich, Germany, succeeded in determining the crystal structure of carbonic acid, hence proving the existence of the molecule once and for all (see Inorganics 2022, 10(9), 132).

The scientists first had to combine the fundamental molecules H₂O (ordinary water) and CO₂ (carbon dioxide, the gas we exhale) at a low temperature (about –100 degrees and even lower), in solid form, that is, “snow” of water and “snow” of carbon dioxide. Then, the mixture was compressed in a specially built apparatus at a pressure of about 20,000 atmospheres; the high-pressure cell consists of, among other things, “Russian Alloy,” a material not available outside Russia, but the composition is known and could be prepared at Aachen. The scientists then hoped that, when squeezed to the extreme, H₂O and CO₂ would combine and form carbonic acid—and the molecules did. The crystal structure of carbonic acid was then scanned with neutron beams at the Garching reactor, Germany, and finally elucidated with a great deal of assistance from computational chemistry, after a total of eight years. Seeing is believing, and we can now see the molecule of carbonic acid—it truly exists.

In the H₂CO₃ crystal, two molecules “pair up” to reach a spatially and energetically better state, and the bonds between the atoms reveal amazing things that can only be explained by complicated methods of numerical quantum chemistry. For example, the carbon–oxygen “single bond” (sharing two electrons between the atoms) is somewhat too short, but the carbon–oxygen “double bond” (sharing four electrons) is somewhat too long, but only at first sight: the molecule, in fact, finds a clever way to optimize its shape. The molecules are then linked to each other with so-called “hydrogen bonds”, which nature also uses to store genetic information in our DNA.

What is the purpose of such fundamental research? There is a plethora of answers, but these are some particularly important ones. By performing the so-called “sequestration” of carbon dioxide into the Earth’s soil, one tries to prevent further entry of CO₂ into the atmosphere. Only now, however, are we in a position to correctly assess the chemical consequences of such action (under the influence of very high pressure). However, carbonic acid probably also plays a role outside our planet: in our solar system, this molecule might be found on other planets (say, Uranus or Neptune and suchlike) and also possibly on Jupiter’s moon Europa. The now confirmed existence of carbonic acid and its structure will make it easier for future scientists to detect the presence of the compound on such celestial bodies, even beyond our solar system. Additionally, almost all chemistry textbooks will have to be rewritten, because the simplest molecule consisting of water and carbon dioxide is not a fantasy; this compound actually exists.