- Names
-
- Wolfram W. Rudolph
- Dieter Fischer
- Gert Irmer
- Title
- Vibrational Spectroscopic Studies and Density Functional Theory Calculations of Speciation in the CO$_2$ Water System
- Abstract
- Raman spectra of $CO_2$ 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 $CO_2$ could be detected. Raman spectra of crystalline $KHCO_3$ and $KDCO_3$ were measured at 22 °C and compared with the infrared data from the literature. In these solids, $(H(D)CO_3)_2^{2–}$ dimers exist and the spectra reveal strong intramolecular coupling. The vibrational data of the dimer (C$_{2h}$ 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 $KHCO_3$, and $KDCO_3$ solutions in $D_2O$ 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 $HCO^3−$(aq) and $DCO^3−$($D_2O$) were assigned, and the measured data compared well with data derived from DFT calculations. The symmetry for $HCO^3−$(aq) is C1, while the gas-phase structure of $HCO^3−$ possesses Cs symmetry. No dimers could be found in aqueous solutions, but at the highest $KHCO_3$ concentration (3.270 mol/kg) intermolecular coupling between HCO3−(aq) anions could be detected. $KHCO_3$ 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 $Na_2$–, $K_2$–, and $Cs_2CO_3$ 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 $K_2CO_3$ solutions were carried out and the hydrolysis degree a was determined as a function of concentration at 22°C. The second dissociation constant, pK$_2$, of the carbonic acid was determined to be equal to 10.38 at 22°C.
- Keywords
- spectroscopy, Raman spectroscopy, Infrared spectroscopy, ATR absorption, absorbance, MIR, band position, band intensity, band attribution, minerals, carbonate, Kalicinite, KHCO3, KDCO3, HCO3- ion, DCO3- ion, water solution
- Content
- band list data, spectral data, experimental physics
- Year
- 2006
- Journal
- Applied Spectroscopy
- Volume
- 60
- Number
- 2
- Pages
- 130 - 144
- Document type
- article
- Publication state
- published
- Doi
- 10.1366/000370206776023421
- Identifiers
-
- bibcode: 2006ApSpe..60..130R