CARBONIC-ACID - SYNTHESIS BY PROTONATION OF BICARBONATE AND FTIR SPECTROSCOPIC CHARACTERIZATION VIA A NEW CRYOGENIC TECHNIQUE

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 vacuo by FTIR spectroscopy from 78 to 300 K. At almost-equal-to 120 K, i.e. almost-equal-to 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 almost-equal-to 140 to almost-equal-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 almost-equal-to 1630 cm-1. Simultaneously formation of a band centered at almost-equal-to 1730 cm-1 (almost-equal-to 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 almost-equal-to 175 K and pumping off first methanol and excess HCl and then residual water. On further heating to almost-equal-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.