Vibrational spectroscopic studies and DFT calculations on NaCH₃CO₂(aq) and CH₃COOH(aq)

Abstract

Aqueous solutions of sodium acetate, NaCH3CO2, and acetic acid, CH3COOH, were studied using Raman and infrared spectroscopy. The spectra were recorded over a large concentration range, in the terahertz region and up to 4000 cm(-1). In the isotropic Raman spectrum in R-format, a polarized band at 189 cm(-1) was assigned to the ν1Na-O stretch of the hydrated Na(+)-ion and a shoulder at 245 cm(-1) to the restricted translation band, νsO-H···O* of the hydrated acetate ion, CH3CO2(-)(aq). The CH3CO2(-)(aq) and the hydrated acetic acid, CH3COOH(aq), possess pseudo Cs symmetry. Geometrical parameters for the species in the gas phase and for CH3CO2(-)(aq) and CH3COOH(aq) are reported. Characteristic bands for CH3CO2(-)(aq) and CH3COOH(aq) were assigned under the guidance of the DFT vibrational frequency calculations and discussed in detail. In aqueous NaCH3CO2 solutions, at high concentrations, no contact ion pairs could be detected, but instead solvent separated ion pairs were found. In LiCH3CO2(aq), however, contact ion pairs are formed which is indicated by the appearance of a shoulder at 939 cm(-1) and the shift of the symmetric stretching mode of the -CO2(-) group to higher wavenumbers.

Intermolecular interactions in mixtures of 1-n-butyl-3-methylimidazolium acetate and water: Insights from IR, Raman, NMR spectroscopy and quantum chemistry calculations

“…The COO − stretching vibrational modes are very sensitive to the chemical environment [76][77][78][79] and, hence, they are excellent probes of the H-bonding interactions. Furthermore, our calculations show that the asymmetric and symmetric carboxylate stretching modes are typically found in the IR spectra at, respectively, 1560 cm − 1 and 1384 cm −1 (for the latter one, there is a strong overlap with the contribution of the CH 3 bending) while in the Raman spectra they are found at 1566 cm −1 and 1389 cm −1 .…”

“…Furthermore, our calculations show that the asymmetric and symmetric carboxylate stretching modes are typically found in the IR spectra at, respectively, 1560 cm − 1 and 1384 cm −1 (for the latter one, there is a strong overlap with the contribution of the CH 3 bending) while in the Raman spectra they are found at 1566 cm −1 and 1389 cm −1 . The splitting between the asymmetric and symmetric vibration modes is often used to determine the geometry and strength of the interaction between the COO − moiety and its direct environment [76][77][78][79]. Concerning this point, the analysis of the symmetric and asymmetric frequencies obtained from our quantum chemical calculations (see Table 1) on the trimer of BmimAc and on the solvated BmimAc shows that (i) there is a strong overlap between the spectral contribution of COO − (both stretching and bending vibrational modes) and that of the N 1 C 2 N 3 of the Bmim + ; (ii) the symmetric stretching vibrational modes of the COO − groups engaged in hydrogen bonding are at higher wavenumbers than those of the free ones (positioned at farther distance with respect to any atoms of the Bmim + ) while the asymmetric stretching vibrational modes of the hydrogen bonded COO − are at lower wavenumbers compared to the free ones; (iii) the closer the COO − moiety is to the acidic hydrogen atoms of the Bmim + (the stronger is the interaction), the smaller is the difference between the symmetric and asymmetric stretching frequencies.…”

“…Another region of interest (see Fig. 11) is the one located between 550 and 700 cm − 1 which encompasses the in plane and out of plane bending modes of the COO − moiety [76,78,79]. In the IR spectra (see Fig.…”

Raman spectroscopic studies and DFT calculations on NaCH3CO2and NaCD3CO2solutions in water and heavy water

“…The acetate water cluster with one water molecule, CH 3 CO 2 À $H 2 O still possesses C s symmetry and the water molecule binds in a bifurcated fashion to the two oxygen atoms of the -CO 2 moiety. 12 and thus allows comparison of the parameters with the ones presented in Table 1 (ref. S1 †).…”

“…7,8 Molecular dynamics (MD) simulations demonstrated, furthermore, that the rst-shell waters are either loosely or tightly bound to their CH 3 CO 2 À oxygen atoms leading to a uctuation in the hydration number ranging from 2-5. 12 In this study, we have expanded our work on acetate in aqueous solution to investigate acetate-d 3 , CD 3 CO 2 À (aq), by including the vibrational deuterium isotope effect of the C-H bands (Teller-Redlich product rule) as one central point of interest. 10,11 A combined Raman and infrared spectroscopic investigation on NaCH 3 CO 2 (aq) was undertaken and for NaCH 3 CO 2 solutions, it was established that even up to very high concentrations, the acetate forms solvent separated ion pairs with Na + instead of contact ion pairs and in addition vibrational band assignments of CH 3 CO 2 À (aq) were supported by comparison with DFT frequencies.…”