A POLYMORPH OF CARBONIC-ACID AND ITS POSSIBLE ASTROPHYSICAL RELEVANCE

Layers of glassy aqueous solutions of HCO3- and of excess HBr have been deposited sequentially on top of each other at 78 K in the form of droplets, and their reaction was studied in vacuo by FTIR spectroscopy from 78 to 300 K. After pumping-off ice, and excess HBr and its hexahydrate, formation of a polymorph of carbonic acid (beta-H2CO3) is observed. On further heating to 250 K, it sublimes and/or decomposes. The polymorph can be converted into the form obtained previously from methanolic solution (alpha-H2CO3). We further show that beta-H2CO3 is the same product reported by Moore and Khanna for proton irradiation of cryogenic CO2-H2O mixtures (Spectrochim. Acta, Part A, 1991, 47, 255). This adds an astrophysical relevance to the properties of carbonic acid, which is discussed for cometary nuclei.