Carbonic acid: Synthesis by protonation of bicarbonate and FTIR spectroscopic characterization via a new cryogenic technique

Article abstract of DOI:10.1021/ja00071a061

Layers of glassy solutions of HCO₃^- (DCO₃^-) and of excess HCl (DCl) dissolved in CH₃OH (CH₃OD) were deposited one by one onto each other at 78 K in the form of droplets, and their reaction has been studied in vacua by FTIR spectroscopy from 78 to 300 K. At ≈120 K, i.e. ≈20 K above the solvents' glass transition temperature of 103 K, a decrease in the solvents' viscosity led to the beginning of the coalescence of the droplets. At ≈=140 to ≈160 K, a further decrease in the solvents' viscosity enabled the reaction Of HCO₃^- (DCO₃^-) with H⁺ (D⁺) as seen most clearly by the disappearance of the bicarbonate band at ≈1630 cm^-1. Simultaneously formation of a band centered at ≈1730 cm^-1 (≈1725 cm^-1) is observed which is in the frequency region expected for a C=O stretching vibration of H₂CO₃ (D₂CO₃). Separation of the reaction product from the solvent was achieved by heating in vacuo up to ≈175 K and pumping off first methanol and excess HCl and then residual water. On further heating to ≈190 K, the reaction product also started to vaporize. We conclude that we have isolated carbonic acid via a novel cryogenic technique and give a preliminary assignment. It is important to note that the reaction can be reversed in an additional step by depositing a layer of KOH in glassy CH₃OH onto the isolated carbonic acid. The new cryogenic technique is particularly suitable for studies of short-lived intermediates in the reaction of nonvolatile reactants such as biomolecules. It is possibly best applied to studies of consecutive reactions where a metastable intermediate is formed in the first step in a preliminary equilibrium and the second step is rate determining.