Vibrational Spectroscopic Studies and Density Functional Theory Calculations of Speciation in the CO 2 –Water System WOLFRAM W. RUDOLPH,* DIETER FISCHER, and GERT IRMER Institut fu ¨ r Virologie im MTZ, TU Dresden, Fetscherstr. 42, D-01307 Dresden, Germany (W.W.R.); Institute of Polymer Research Dresden, Hohe Strasse 6, D-01069 Dresden, Germany (D.F.); and Institut fu ¨r theoretische Physik, TU Bergakademie Freiberg, Leipziger Str. 23, D-09596, Freiberg, Germany (G.I.) Raman spectra of CO 2 dissolved in water and heavy water were measured at 22 8C, and the Fermi doublet of CO 2 , 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 8C 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 2 O 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 2 O) were assigned, and the measured data compared well with data derived from DFT calculations. The symmetry for HCO 3 – (aq) is C 1 , while the gas-phase structure of HCO 3 – possesses C s symmetry. No dimers could be found in aqueous solutions, but at the highest KHCO 3 concentration (3.270 mol/kg) intermolecular coupling between HCO 3 – (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 2 CO 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 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 K 2 CO 3 solutions were carried out and the hydrolysis degree a was determined as a function of concentration at 22 8C. The second dissociation constant, pK 2 , of the carbonic acid was determined to be equal to 10.38 at 22 8C. Index Headings: Alkali metal carbonates; Hydrogen carbonate; Carbon dioxide; Fourier transform infrared spectroscopy; FT-IR; Raman; Aqueous solution chemistry; Density functional theory; DFT. INTRODUCTION The dissociation of carbon dioxide in water and the subsequent equilibrium processes involving carbonate and bicarbonate ions have been the subject of many publications. 1–7 These studies reflect the importance of the role of the CO 2 – H 2 O equilibria in governing chemical, environmental, and biological processes in natural waters. 1 The bicarbonate is the most important buffer of the blood and therefore of utmost medical importance. Although considerable data have been collected on the kinetic and thermodynamic constants of the system, there is little information on the structure of these aqueous species. 8–10 An aqueous solution of CO 2 shows a weak acidic pH value because the CO 2 reacts slightly with water (;0.2% at room temperature) and forms carbonic acid: CO 2 ðaqÞþ H 2 O $ H 2 CO 3 ðaqÞþ H 2 O $ HCO  3 þ H 3 O þ ð1Þ This acid, which has not been isolated in aqueous solution (but exists in solid state and in the gas phase 7 ), is, in a theoretical sense, a medium strong acid and its dissociation constant K 1 ¼ C Hþ C HCO3– /C H2CO3 would be ;1.310 4 (pK 1 ; 3.88). However, because ;99.8% of the dissolved carbon dioxide does not constitute as H 2 CO 3 but as dissolved CO 2 , best described as CO 2 (aq), the whole solution reacts only slightly acidic. Therefore, it is customary to state the ‘‘apparent dissociation constant’’ by substituting the undissociated fraction of the acid content as C H2CO3 þCO2 , and subsequently K 1 is about three orders of magnitude smaller, namely, 4.4510 7 (pK 1 ¼ 6.35), and therefore is commonly classified as a weak acid. The second dissociation step of the carbonic acid is, in contrast to the first step (on the order of seconds), very fast: HCO  3 þ OH  $ CO 2 3 þ H 2 O ð2Þ and for K 2 follows, K 2 ¼ C Hþ C CO3 2 =C HCO3 is 4.8410 11 (pK 2 ¼ 10.33). The determination of the content of H 2 CO 3 (aq), respectively, HCO 3 – þ H 3 O þ in an aqueous CO 2 solution, is possible because the addition of a strong base instantly neutralizes an acid such as H 2 CO 3 (aq), while further neutralization takes time because CO 2 þ OH – $ HCO 3 – is a slow process. On the other hand, H 2 CO 3 (aq) þ OH – $ CO 3 2– þ H 2 O is very fast. 1 Speciation studies in aqueous solutions are, to our knowledge, sparse, with only one publication dealing with the spectroscopic characterization of speciation in hydrogen carbonate solutions, 8 while carbonates have been studied more frequently. Due to methodological limitations at the time, 8–10 only fairly concentrated HCO 3 – solutions, 10 CO 3 2– solutions, and hydrate carbonate melts, as well as anhydrous carbonate melts, 9,10 were studied. Recently, we published Raman investigations on mineral waters with the aim of achieving qualitative and quantitative information on the species present, including hydrogen carbonate and CO 2 (aq) 11 among other anions. Our results spurred the present systematic study in which we report on the Raman and infrared (IR) spectroscopic investigations of speciation in solutions of carbon dioxide, hydrogen carbonate Received 14 November 2005; accepted 20 December 2005. * Author to whom correspondence should be sent. E-mail: wolfram. rudolph@tu-dresden.de. Volume 60, Number 2, 2006 APPLIED SPECTROSCOPY 0003-7028/06/6002-000000$2.00/0 Ó 2006 Society for Applied Spectroscopy