Until now, it has stubbornly defied all attempts at detection: according to current textbooks, gas-phase carbonic acid should not exist at all, because it immediately decomposes into water and carbon dioxide –making it detectible only as a trace component.

A team led by Hinrich Grothe at the TU Vienna and Thomas Loerting at the University of Innsbruck has now finally found proof to the contrary.

As the Austrian researchers report in the journal Angewandte Chemie, they were able to isolate carbonic acid and gather spectroscopic data.

Carbonated beverages contain carbon dioxide. They also contain trace amounts of a molecule that was long thought to be too unstable to exist: carbonic acid (H₂CO₃). It is now known that carbonic acid is indeed present in drinks, though at very, very low concentrations. Until recently, the molecule has resisted all attempts at isolation and direct detection. However, a few scientists have been able to produce carbonic acid in the solid state. It is also assumed to be present in cirrus clouds in Earth’s atmosphere and in space.

The Austrian researchers have now demonstrated that carbonic acid can exist in the gas phase and that it is stable at temperatures up to –30 °C. For these experiments, solid carbonic acid was formed by means of acid-base reactions at very low temperatures and then warmed to –30 °C. The evaporating molecules were trapped in a matrix of the noble gas argon and then immediately cooled again. This resulted in a kind of frozen “image” of the gas-phase carbonic acid, which the researchers were able to study by infrared spectrometry.

The spectra showed that gas-phase carbonic acid exists in three different forms. The scientists found two monomers that differ in their conformation - the spatial arrangement of their atoms - as well as a dimer made from two molecules bound through hydrogen bonds.

The resulting detailed spectrometric data are of great interest to astronomers, because they could make it easier to detect gas-phase carbonic acid in space, where it is thought to be present in the tails of comets and on Mars.

Press release: University of Innsabruck, Austria:

Gas-phase Carbonic Acid Isolated

A team of chemists headed by Thomas Loerting from the University of Innsbruck and Hinrich Grothe from the Vienna University of Technology (TU Wien) in Austria have prepared and isolated gas-phase carbonic acid and have succeeded in characterizing the gas-phase molecules by using infrared spectroscopy. The results were published in the journal Angewandte Chemie International Edition.

In textbooks and other media the widespread belief still prevails that stable carbonic acid cannot be produced in pure form and is practically non-existent as it immediately decomposes to carbon dioxide and water. However, Innsbruck chemists headed by Erwin Mayer (Institute of General, Inorganic and Theoretical Chemistry) refuted this persistent dogma in chemistry several years ago. They belong to only a handful of scientists who have prepared pure solid carbonic acid experimentally. In an international first, the scientists have now produced gas-phase carbonic acid and, together with a research group headed by Hinrich Grothe at the Vienna University of Technology, they have also succeeded in proofing the existence of these molecules. “Carbonic acid vapor is composed of at least three different species in the gas-phase: a cyclic dimer consisting of two molecules and two different types of monomers,“ explains Thomas Loerting (Institute of Physical Chemistry) the result of the comprehensive study.

Surprising result

For this experiment the researchers prepared carbonic acid in the laboratory in Innsbruck. It was then stored in liquid nitrogen and transported to Vienna by PhD student Jürgen Bernard. At the Institute of Materials Chemistry at the TU Wien the solid carbonic acid was warmed to minus 30 degrees Celsius. “During this process the carbonic acid molecules entered the gas-phase,“ says Loerting. This is a surprising result because many experts in the field believed that carbonic acid immediately decomposes to carbon dioxide and water. The Austrian scientists trapped the carbonic acid vapor in a solid matrix of the inert gas argon and cooled it down. “This produced a frozen image of the carbonic acid vapor, which we analyzed by using high-resolution infrared spectroscopy at the TU Wien,“ says Hinrich Grothe. “The spectrum we produced is extremely precise and we were able to assign the spectral bands to the vibration of each single molecule.“ For more than a decade, the chemists have been supported in their experimental research by Klaus Liedl from the Institute of Theoretical Chemistry in Innsbruck. His team of scientists has helped to interpret the experimental data with computational models. Additional calculations have been performed by Oscar Galvez from CSIC Madrid (Spanish National Research Council).

Infrared spectra in research

This experiment not only is of high importance for basic research but also for astronomy. The identification of gas-phase carbonic acid in the atmosphere of celestial bodies may be facilitated by the detailed spectra of gas-phase carbonic acid described in this study. “Conditions in space environments suggest that gas-phase carbonic acid may be found in the coma of comets or the poles of Mars,“ says Thomas Loerting. “However, infrared spectra currently measured in extraterrestrial environments are still too imprecise to be comparable to the results produced in our laboratory.“

The team of chemists headed by Loerting and Liedl are members of the research platform Advanced Materials of the University of Innsbruck and are supported by the Austrian Science Fund (FWF) and the European Research Council (ERC). The chemists in Vienna supervised by H. Grothe participate in the TU Wien Materials Research Cluster and are supported by the Austrian Exchange Service (ÖAD).