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 (β-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 (α-H2CO3). We further show that β-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.