Carbonic Acid Crystal Forms Clarified

Carbonic acid, H₂CO₃, is a key molecule in both biology and geology. In your body, it plays an intermediate role in transporting carbon dioxide, and the equilibrium between carbonic acid and bicarbonate buffers your blood to keep it at about pH 7.4. In the environment, CO₂ taken up by water forms carbonic acid, which is then responsible for dissolution of carbonate minerals and ocean acidification. Astrochemists expect that carbonic acid exists in solar system ices, such as those on Mars, and in interstellar regions.

But despite being ubiquitous, carbonic acid has been difficult to study as a pure substance because it readily dissociates into CO₂ and H₂O. Researchers in the 1990s identified two crystalline polymorphs, christened the α and β forms, thought to differ by how the molecules hydrogen bond to each other. The substances, however, eluded precise structural determination. Now, new infrared spectra of carbonic acid trapped from the gas phase demonstrate that so-called α-H₂CO₃ is likely the monomethyl ester, or methyl hydrogen carbonate, CH₃OCO₂H (Angew. Chem. Int. Ed. 2014, DOI: 10.1002/anie.201406969)

These results will help researchers monitor carbonic acid and understand its roles in the chemistry of Earth’s atmosphere and interstellar space, says Markku Räsänen, a chemistry professor at Finland’s University of Helsinki. The findings also show “the power of the combination of computational and matrix isolation spectroscopy in finding essential species,” Räsänen adds, noting that he has also tried—unsuccessfully—to make carbonic acid.

“The work highlights that it’s very, very important to avoid misidentification by doing experiments under well-defined and clean conditions to avoid artifacts,” adds Ralf I. Kaiser, a chemistry professor at the University of Hawaii, Manoa, who has also worked on carbonic acid.

Previously, β-H₂CO₃ was typically prepared by irradiating cryogenic ice mixtures of CO₂ and H₂O or by protonating bicarbonate or carbonate in aqueous solution also under cryogenic conditions. α-H₂CO₃ was prepared via protonation in methanolic solution. But studies of α- and β-H₂CO₃ in which the substances were sublimated and then trapped in a noble-gas matrix showed differences in the IR spectra of molecules from the two materials (J. Am. Chem. Soc. 2013, DOI: 10.1021/ja4020925). That work was done by a team in Austria led by graduate student Jürgen Bernard and chemistry professor Thomas Loerting of the University of Innsbruck and Hinrich Grothe of Vienna University of Technology. The researchers also asserted that α-H₂CO₃ sublimes and recrystallizes as α while β-H₂CO₃ sublimes and recrystallizes as β.

Those results piqued the interest of Peter R. Schreiner, a professor of chemistry at Justus Liebig University, in Giessen, Germany. “You’d expect the infrared spectra of two polymorphs to be identical, yet they show two different carbonyl absorptions,” Schreiner says. “And then, if they’re condensed in the same way, they should wind up in the same form. When I saw this paper, I said that we have to get on this, we have to make carbonic acid in an independent way.”

Schreiner, senior scientist Hans Peter Reisenauer, and graduate student J. Philipp Wagner designed an approach to prepare carbonic acid through pyrolysis of alkyl carbonates followed by trapping the products in a noble-gas matrix for spectroscopic study. The researchers found that the IR spectrum of carbonic acid prepared through pyrolysis matched that of β-H₂CO₃. They could only reproduce the spectrum of α-H₂CO₃, however, by pyrolysis of tert-butyl methyl carbonate to form CH₃OCO₂H.

Loerting agrees that IR spectra indicate the presence of a methyl group in both his group’s α-H₂CO₃ and the pyrolysis samples. Bernard’s thesis, published earlier this year, also acknowledges the possibility of the monomethyl ester. But Loerting questions whether the monomethyl ester is the primary product or a side product. For a definitive answer, he would like to see mass spectrometry experiments show CH₃OCO₂H in solid α samples, but he acknowledges the difficulty of such studies, because both species tend to fragment upon ionization.