Characterization of Carbonic Acid (H2CO3)

Synthesis and characterization of carbonic acid (H2 CO 3 ) has been an elusive task until the early nineties. In fact, the textbook opinion was that it can not be isolated at all because of its rapid decomposition to carbon dioxide and water. In 1993 our group was for the first time able to isolate H 2 CO 3 in its solid form using a cryo-technique and characterize it by means of FT-IR spectroscopy [Hage et al., J. Am. Chem. Soc., 115 (1993) 8427]. Meanwhile, comparison of laboratory spectra with spectra of stellar bodies has indicated the possibility of the prevalence of H2 CO 3 on the Martian surface, on Galilean satellites of Jupiter, comets like Halley and Edgeworth-Kuiper-Belt bodies. On adapting the cryo-technique slightly by changing from aqueous to methanolic solution we were able to characterize a second crystalline polymorph of H2 CO 3 . A major aim of the proposal at hand is to investigate whether other polymorphs can be generated by changing the solvent or using mixtures of solvents. The two known polymorphs as well as any polymorphs to be discovered will be characterized for the first time not only in situ by FT-IR spectroscopy, but also ex situ by Raman spectroscopy, differential scanning calorimetry and powder X-ray diffraction. The latter method opens the possibility of a crystal structure determination, which will be assisted by theoretical crystal structure predictions in the group of Prof. Sarah L. Price (UCL). Besides its astrophysical relevance in the solid state there is also speculation that gaseous carbonic acid forms in the atmosphere of Venus and also in Earth`s atmosphere. For a verification/falsification of this claim spectra of gaseous carbonic acid are required. Our succesful sublimation and recondensation experiment from 1998 for a carbonic acid polymorph [Hage et al., Science, 279 (1998) 1332] opens the possibility of measuring such spectra in the laboratory. While it is not possible to detect carbonic acid directly in the gas-phase, the matrix isolation technique allows to record spectra of immobilized gas-phase species at low temperatures. The aim is, therefore, to record spectra of carbonic acid in an Argon matrix at low temperatures.

Carbonic acid (H2 CO 3 ) was long thought to be non-existent as a solid, and only very short-lived in the gas-phase and in aqueous solutions. This erroneous opinion still prevails in some contemporary textbooks used for teaching chemistry students. In the last 20 years, though, a few research groups have succeeded in isolating solid carbonic acid. A NASA group and a group in Sicily have succeeded in preparing solid carbonic acid by high-energy irradiation of carbon dioxide or carbon dioxide/water mixtures. Since conditions in space were mimicked as closely as possible by these groups, the hypothesis of carbonic being present in astrophysical environments was born. Our Innsbruck group has succeeded by the use of a low-temperature acid-base reaction in the preparation of even two different crystalline solid modifications of carbonic acid, called the a-polymorph and the ß-polymorph. It turned out that the high-energy irradiation product is identical to our ß-polymorph. While mid infrared spectra of the ß- polymorph were available before the start of the present project, a unique identification of carbonic acid in space has not been possible since then. However, in particular the poles of Mars and comets such as Halley have been determined to be likely candidates for the "natural" occurrence of carbonic acid. In the present project we have shown that solid carbonic acid can not only occur in two crystalline forms, but also in two non-crystalline, glassy forms, and in the form of a hydrate. We report Raman spectra and X-ray diffractograms of all these materials in the hope for a possible detection of solid carbonic acid, e.g., by the Mars Microbeam Raman Spectrometer MMRS, which is designed to record Raman spectra of the Mars surface. The powder X-ray diffractograms, Raman and FTIR spectra serve as the basis for solving the crystal structure of the two crystalline forms in the future. Furthermore, we show that natural carbonic acid may not only exist in astrophysical environments, but also in Earth`s atmosphere. We show that carbonate-containing mineral dust particles, such as calcite, floating in Earth`s atmosphere may experience an acid-base reaction, and that carbonic acid may be the stable product of this reaction at temperatures between -70C and -20C. We finally show that even gas-phase carbonic acid is stable by recording high-resolution infra-red spectra of the gas-phase species trapped in an inert solid matrix of noble gases. These matrix spectra, in combination with calculations, provide the basis for understanding conformational equilibria in H 2 CO 3 and the existence of dimeric carbonic acid (H2 CO 3 ) 2 .