Metastable intermediates from glassy solutions .4. FTIR spectra of beta-carbonic acid and its H-2 and C-13 isotopic forms, isolated from aqueous solution

Layers of glassy aqueous H2O (D2O) solutions of (KHCO3)-C-12 ((K2CO3)-C-12, (K2CO3)-C-13) 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 (H2CO3)-C-12 ((D2CO3)-C-12, (H2CO3)-C-13), i.e. beta-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 beta-H2CO3 on heating to 220 K. Effects of protonating agent, base pressure and methanol treatment of a beta-H2CO3 film are discussed with respect to the amorphous --> crystalline H2CO3 phase transition. Sublimation of beta-H2CO3 gives a deposit whose FTIR spectrum resembles that of alpha-H2CO3. Assignment for the spectral features of beta-H2CO3 and the H-2 and C-13 isotopic forms is given and, based on comparison of the O--H-stretching band regions, we conclude that hydrogen bonding is stronger in alpha- than in beta-H2CO3.