Metastable intermediates from glassy solutions. Part 4.—FTIR spectra of β-carbonic acid and its 2H and 13C isotopic forms, isolated from aqueous solution
Layers of glassy aqueous H2O (D2O) solutions of KH12CO3(K212CO3, K213CO3) and of excess HBr (DBr) or of excess HCl have been deposited sequentially at 78 K in the form of droplets, and their reaction has been studied by FTIR spectroscopy (from 4000 to 400 cm–1) from 78 to 300 K. Protonation of HCO3–/CO32– apparently occurs in the freeze-concentrated state, after crystallization of glassy water to cubic ice. When using HBr, crystalline H212CO3(D212CO3, H213CO3), i.e.β-H2CO3, is immediately isolated at 200 K, after pumping off ice and excess HBr. When using HCl, amorphous H2CO3 is first isolated at 200 K and crystallizes to β-H2CO3 on heating to 220 K. Effects of protonating agent, base pressure and methanol treatment of a β-H2CO3 film are discussed with respect to the amorphous → crystalline H2CO3 phase transition. Sublimation of β-H2CO3 gives a deposit whose FTIR spectrum resembles that of α-H2CO3. Assignment for the spectral features of β-H2CO3 and the 2H and 13C isotopic forms is given and, based on comparison of the O—H stretching band regions, we conclude that hydrogen bonding is stronger in α-than in β-H2CO3.