On the search for C–H–O hydrogen bonding in aqueous acetic acid: Combined high-pressure infrared spectroscopy and ab initio calculations study

FTIR spectroscopy and molecular level insight of diluted aqueous solutions of acetic acid

Aqueous solutions of acetic acid (AA) have been intensively explored for decades with a particular attention addressed to the hydrogen bond network generated by COOH group at different concentrations. In majority of studies conducted so far the envelope originated from νCO is decomposed into two bands assigned to differently hydrated monomers: the one presumably to AA···H2O, and another one to AA···(H2O)2. In order to examine if species other than the mentioned monomers produce this spectral signature, we performed computational and FTIR spectroscopic study of AA in aqueous solutions. Dilute solutions of deuterated acetic acid (CD3COOD) in D2O and in C2Cl4 as a reference were prepared (c0 = 0.001, 0.01 and 0.1 mol dm-3) as well as of deuterated sodium acetate (CD3COONa) in D2O. CD3COOD in 0.1 mol dm-3 solution in D2O displays a feature that separated in two signals with maxima at 1706 cm-1 and 1687 cm-1. A combined DFT and molecular dynamics study performed in this work showed the assignation of those spectral bands to be a more complex problem than previously thought, with syn-anti isomerism and hydration contributing to the experimentally observed broad νCO envelope.

Application of Raman multivariate curve resolution to solvation-shell spectroscopy

Raman spectroscopy and multivariate curve resolution (Raman-MCR) are combined to yield a powerful spectroscopic method for identifying solute-induced perturbations of solvent molecules. The principles and applications of the resulting solvation-shell spectroscopy are described and illustrated using both numerical model spectra and experimental Raman spectra, including water in acetone and aqueous OH(-), as well as of both neutral and ionic acetic acid solutions. The results illustrate the quantitative capabilities of Raman-MCR as a solvation-shell spectroscopy, including fundamental limitations arising from "intensity" and "rotational" ambiguities.

C[bond]H- - -O hydrogen bonds in beta-sheetlike networks: combined X-ray crystallography and high-pressure infrared study

Close interactions of the C(alpha)[bond]H- - -O type have been analyzed via X-ray crystallography and high-pressure infrared spectroscopy. The results demonstrate that the C(alpha)[bond]H- - -O interactions can offer an additional stability to the beta-sheet formation. X-ray structural data suggest that while 1-acetamido-3-(2-pyrimidinyl)-imidazolium bromide exhibits a bilayer stacking, the PF(6)(-) salt reveals a beta-sheetlike pattern. The appearance of the free-NH infrared absorption indicates that the conventional N[bond]H- - -O or N[bond]H- - -N hydrogen bonds do not fully dominate the packing for the PF(6)(-) salt. The high-pressure infrared study suggests that the C(alpha)[bond]H- - -O hydrogen bonds are the important determinants for the stability of the PF(6)(-) salt. This study also verifies that the imidazolium C[bond]H stretching frequency shifts to a longer wavelength upon the formation of the C[bond]H- - -O hydrogen bonds.