Metastable intermediates from glassy solutions part 5: FTIR spectroscopic characterization of isolated α- and β-carbonic acid

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 2H 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.