Metastable intermediates from glassy solutions. Part 3.—FTIR spectra of α-carbonic acid and its 2H and 13C isotopic forms, isolated from methanolic solution

Abstract

Layers of glassy solutions of KH¹²CO₃(KD¹²CO₃, K₂¹³CO₃) and of excess HCl (DCl) dissolved in CH₃OH (CH₃OD) and in 70 wt.% glycerol–water have been deposited sequentially at 78 K in the form of droplets, and their reaction in the temperature range 78–300 K studied by FTIR spectroscopy (from 4000 to 400 cm^–1). At ca. 20 K above the glass-transition temperature (T_g) of the solvent, a decrease in viscosity initiated coalescence of the droplets. At ca. 40 K above T_g, protonation of the hydrogen carbonate (carbonate) and formation of H₂CO₃ occurred in the liquid state. After pumping off the solvent, residual ice and excess HCl, amorphous H₂¹²CO₃(D₂¹²CO₃, H₂¹³CO₃) was isolated, crystallizing at ca. 190–200 K to α-carbonic acid. Assignments for the spectral features of α-H₂CO₃ and its isotopic forms are given. Base pressure, water content and heating rate lead to varying degrees of disorder in the crystalline phase. The cryogenic method for formation of metastable short-lived intermediates from reaction of non-volatile reactants is discussed. The intermediates can be studied in solution or as amorphous or crystalline solids.