13 C NMR as an analytical tool for the detection of carbonic acid and pKa determination.

NMR spectroscopy has become a standard technique in studies both on carbon capture and storage. 13 C NMR allows the detection of two peaks for carbonated aqueous samples: one for CO2(aq) and another one for the species H2 CO3 , HCO3 - , and CO3 2- -herein collectively named Hx CO3 x-2 . The chemical shift of this second peak depends on the molar fraction of the three species in equilibrium and has been used to assess the equilibrium between HCO3 - and CO3 2- . The detection of H2 CO3 at low pH solutions is hindered, because of the concurrent liberation of CO2 when the medium is acidified. Herein, a valved NMR tube facilitates the detection of the Hx CO3 x-2 peak across a wide pH range, even at pH 1.8 where the dominant species is H2 CO3 . The method employed the formation of frozen layers of NaH13 CO3 and acid solutions within the tube, which are mixed as the tube reaches room temperature. At this point, the tube is already securely sealed, preventing any loss of CO2 to the atmosphere. A spectrophotometry approach allowed the measurement of the actual pH inside the pressurized NMR tube. The chemical shift for H2 CO3 was determined as 160.33 ± 0.03 ppm, which is in good agreement with value obtained by DFT calculations combined with Car-Parrinello molecular dynamics. The H2 CO3 pKa value determined by the present method was 3.41 ± 0.03, for 15% D2 O aqueous medium and 0.8 mol/L ionic strength. The proposed method can be extended to studies about analogs such as alkyl carbonic and carbamic acids.

Understanding the nature of bonding interactions in the carbonic acid dimers.

Carbonic acid dimer, (CA)2, (H2CO3)2, helps to explain the existence of this acid as a stable species, different to a simple sum between carbon dioxide and water. Five distinct, well characterized types of intermolecular interactions contribute to the stabilization of the dimers, namely, C=O⋯H-O, H-O⋯H-O, C=O⋯C=O, C=O⋯O-H, and C-O⋯O-H. In many cases, the stabilizing hydrogen bonds are of at least the same strength as in the water dimer. We dissect the nature of intermolecular interactions and assess their influence on stability. For a set of 40 (H2CO3)2 isomers, C=O⋯H-O hydrogen bonds between the carbonyl oxygen in one CA molecule and the acidic hydrogen in the hydroxyl group at a second CA molecule are the major stabilizing factors because they exhibit the shortest interaction distances, the largest orbital interaction energies, and the largest accumulation of electron densities around the corresponding bond critical points. In most cases, these are closed-shell hydrogen bonds, however, in a few instances, some covalent character is induced. Bifurcated hydrogen bonds are a common occurrence in the dimers of carbonic acid, resulting in a complex picture with multiple orbital interactions of various strengths. Two anti-anti monomers interacting via the strongest C=O⋯H-O hydrogen bonds are the ingredients for the formation of the lowest energy dimers. Graphical Abstract Carbonic acid dimer, (CA)2, (H2CO3)2, helps explaining the existence of this acid as a stable species, different to a simple sum between carbon dioxide and water. Five distinct, well-characterized types of intermolecular interactions contribute to the stabilization of the dimers, namely, C=O⋯H-O, H-O⋯O-H, C=O⋯C=O, C=O⋯O-C, and C-O⋯O-C. In many cases, the stabilizing hydrogen bonds are of at least the same strength as in the water dimer.

Formation of isomers of anionic hemiesters of sugars and carbonic acid in aqueous medium.

Hemiesters of carbonic acid can be freely formed in aqueous media containing HCO3(-)/CO2 and mono- or poly-hydroxy compounds. Herein, (13)C NMR spectroscopy was used to identify isomers formed in aqueous solutions of glycerol (a prototype compound) and seven carbohydrates, as well as to estimate the equilibrium constant of formation (Keq). Although both isomers are formed, glycerol 1-carbonate corresponds to 90% of the product. While fructose and ribose form an indistinct mixture of isomers, the anomers of d-glucopyranose 6-carbonate correspond to 74% of the eight isomers of glucose carbonate that were detected. The values of Keq for the disaccharides sucrose (4.3) and maltose (4.2) are about twice the values for the monosaccharides glucose (2.0) and fructose (2.3). Ribose (Keq = 0.89)-the only sugar without a significant concentration of a species containing a -CH2OH group in an aqueous solution-resulted in the smallest Keq. On the basis of the Keq value and the concentrations of HCO3(-) and glucose in blood, one can anticipate a concentration of 2-4 µmol L(-1) for glucose 6-carbonate, which corresponds to ca. of 10% of its phosphate counterpart (glucose 6-phosphate).