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Rudolph, W.W. ; Fischer, D. ; Irmer, G.
Vibrational Spectroscopic Studies and Density Functional Theory Calculations of Speciation in the CO2 - Water System

Raman spectra of CO2 dissolved in water and heavy water were measured at 22 °C, and the Fermi doublet of CO2, normally at 1285.45 and 1388.15 cm−1 in the gaseous state, revealed differences in normal water and heavy water, although no symmetry lowering of the hydrated CO2 could be detected. Raman spectra of crystalline KHCO3 and KDCO3 were measured at 22 °C and compared with the infrared data from the literature. In these solids, (H(D)CO3)22− dimers exist and the spectra reveal strong intramolecular coupling. The vibrational data of the dimer (C2h symmetry) were compared with the values from density functional theory (DFT) calculations and the agreement is fair. Careful measurements were made of the Raman spectra of aqueous KHCO3, and KDCO3 solutions in D2O down to 50 cm−1 and, in some cases, down to very low concentrations (≥0.0026 mol/kg). In order to complement the spectroscopic assignments, infrared solution spectra were also measured. The vibrational spectra of HCO3(aq) and DCO3(D2O) were assigned, and the measured data compared well with data derived from DFT calculations. The symmetry for HCO3(aq) is C1, while the gas-phase structure of HCO3 possesses Cs symmetry. No dimers could be found in aqueous solutions, but at the highest KHCO3 concentration (3.270 mol/kg) intermolecular coupling between HCO3(aq) anions could be detected. KHCO3 solutions do not dissolve congruently, and with
increasing concentrations of the salt increasing amounts of carbonate could be detected. Raman and infrared spectra of aqueous Na2 −, K2 −, and Cs2CO3 solutions in water and heavy water were measured down to 50 cm−1 and in some cases down to extremely low concentrations (0.002 mol/kg) and up to the saturation state. For carbonate in aqueous solution a symmetry breaking of the D3h symmetry could be detected similar to the situation in aqueous nitrate solutions. Strong hydration of carbonate in aqueous solution could be detected by Raman spectroscopy. The hydrogen bonds between carbonate in heavy water are stronger than the ones in normal water. In sodium and potassium carbonate solutions no contact ion pairs could be detected even up to the saturated solutions. However, solvent separated ion pairs were inferred in concentrated solutions in accordance with recent dielectric relaxation spectroscopy (DRS) measurements. Quantitative Raman measurements of the hydrolysis of carbonate in aqueous K2CO3 solutions were carried out and the hydrolysis degree a was determined as a function of concentration at 22 °C. The second dissociation constant, pK2, of the carbonic acid was determined to be equal to 10.38 at 22 °C.

Applied Spectroscopy 60



Erschienen am
April 2006

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