Metastable intermediates from glassy solutions .5. FTIR spectroscopic characterization of isolated alpha- and beta-carbonic acid

Layers of glassy aqueous or methanolic solutions of (KHCO3)-C-12 ((K2CO3)-C-13) 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 HCO3-/CO32- and formation of H2CO3 occurred in the liquid phase. After pumping off the solvent, residual ice and excess HCl, amorphous (H2CO3)-C-12 ((H2CO3)-C-13) was isolated, crystallizing at ca. 190-200 K to alpha-carbonic acid. In the case of water as solvent, protonation of HCO3-/CO32- 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 beta-H2CO3. This polymorphic form is the same product reported by Moore and Khanna for proton irradiation of cryogenic CO2-H2O 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. (C) 1997 Elsevier Science B.V.