Abstract
Layers of glassy aqueous or methanolic solutions of KH 12CO 3 (K 13 2CO 3) and of excess HCl or HBr have been deposited sequentially at 78 K in the form of droplets, and their reaction in the temperature range from 78 to 300 K studied by FTIR spectroscopy in the spectral range from 4000 to 400 cm −1. When using methanol as solvent, at ca. 20 K above its glass-transition temperature ( T g ), a decrease in viscosity initiated coalescence of the droplets. At ca. 40 K above T g , protonation of HCO − 3 CO 2− 3 and formation of H 2CO 3 occurred in the liquid phase. After pumping off the solvent, residual ice and excess HCl, amorphous H 12 2CO 3 (H 13 2CO 3) was isolated, crystallizing at ca. 190–200 K to α-carbonic acid. In the case of water as solvent, protonation of HCO − 3 CO 2− 3 apparently occurs in the freeze-concentrated state, after crystallization of glassy water to cubic ice. Reaction in freeze-concentrated aqueous solutions leads to formation of β-H 2CO 3. This polymorphic form is the same product reported by Moore and Khanna for proton irradiation of cryogenic CO 2H 2O mixtures (Spectrochim. Acta Part A, 1991, 47, 255). The cryogenic method is discussed for formation of metastable short-lived intermediates from reaction of non-volatile reactants, where the intermediates can be studied in solution or as amorphous or crystalline solids.
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