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

Abstract

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^–/CO₃^2– 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.