Vibrational Spectroscopic Studies and Density Functional Theory Calculations of Speciation in the CO 2 —Water System

Abstract

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)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 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 C_s 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 D_3h 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.