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J. Bernard, M. Seidl, T. Loerting, I. Kohl, K. Liedl, E. Mayer, O. Galvez, H. Grothe:
"Spectroscopic observation of matrix-isolated carbonic acid trapped from the gas phase";
Talk: Matrix2011, UBC Vancouver, Canada; 2011-07-10 - 2011-07-15; in: "Matrix2011", (2011).

Carbonic acid (H2CO3) has so far eluded most attempts at isolation and direct detection. Despite the widespread belief that it is a highly instable molecule, the pure solid could be prepared previously,[2-4] and it is thought that solid carbonic acid exists in cirrus clouds on Earth and in astrophysical environments.[5] Gas-phase carbonic acid was long thought to immediately decompose to water and carbon dioxide, and therefore to be non-existent or
detectable only as a trace component.[6] We here show that gas-phase carbonic acid is stable at temperatures above 200 K and trap carbonic acid vapor in an inert matrix at 6 K. Spectroscopic analysis of this matrix reveals that carbonic acid vapor is composed of at
least three species: two monomeric conformers and the cyclic dimer (H2CO3)2, and carbon dioxide and water are minor components. The molar ratio of the two monomers suggests that the cis-cis monomer is the most stable one, and the cis-trans monomer is less stable by
~4 kJ/mol, in accordance with theoretical predictions.[7] The stability of gas-phase carbonic acid at temperatures above 200 K suggests that carbonic acid may sublime in astrophysical
environments without decomposition, e.g., on the poles of Mars or in the coma of comets such as Hale-Bopp, and, therefore, our infrared spectra represent a benchmark for possible identification of naturally occurring carbonic acid vapor.

http://www2.chem.ubc.ca/Matrix2011/Abstracts/Grothe.pdf