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J. Bernard, M. Seidl, I. Kohl, K. Liedl, E. Mayer, O. Galvez, H. Grothe, T. Loerting:
"Spectroscopic Observation of Gas-Phase Carbonic Acid Isolated in Matrix";
Angewandte Chemie - International Edition, 50 (2011), 8; 1939 - 1943.

Carbonic acid (H2CO3) is of fundamental importance, e.g., for regulation of blood pH, for ocean acidification or dissolution of carbonates. This six-atom molecule commonly found in carbonated drinks at sub-micromolar concentration 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,[1-5] and it is thought that solid carbonic acid exists in cirrus clouds on Earth and in astrophysical environments.[6-10] 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.[11] 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.[12,13] 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://dx.doi.org/10.1002/anie.201004729