We have investigated water highly diluted in 1-alkyl-3-methyl imidazolium ionic liquids (ILs) with hexafluorophosphate {PF(6)(-)} and tetrafluoroborate {BF(4)(-)} anions using vibrational spectroscopic measurements in the nu(OH) spectral domain of water (3600-3800 cm(-1)) and DFT calculations. The measured profiles exhibit two well-defined bands at coinciding vibrational transitions assigned with the nu(1) symmetric and nu(3) antisymmetric OH stretching modes of monodispersed water. The local organization and the vibrational spectra of water diluted in ILs have been assessed by DFT calculations (using the B3LYP functional and 6-31+G** basis set). We show that the predicted structures of water interacting (minimally) with two anions in nearly "symmetric" structures of type (A...H-O-H...A) lead to spectral features consistent with the previous spectroscopic observations as well as with those reported here. We emphasize the role of the non additive interaction forces (especially the 3-bodies electrostatic interactions) in the structural organization taking place between the cation-anion couples and for determining preferentially (A...H-O-H...A) associations of water with the anions as well as their consequences on the vibrational spectra of water. We show that the doubly hydrogen-bonded character of water in such associations leads to well-defined spectral features, which are the shifts of the nu(1) and nu(3) stretching modes of water, the separation Delta nu(13) between them (about 80 cm(-1)), and the intensity ratio estimates R = I nu(3)/I nu(1) (IR absorption and Raman). Finally, we evoke the fact that the H-bond interactions of water diluted in these ILs involve a more noticeable electrostatic character than for H-bond interactions of water in usual molecular solvents. In this context, we emphasize that the appearance of the Raman band of the nu(3) mode of water originates from a significant polarization of water due to the local electrostatic fields induced by surrounding ions.

The unusual solubility of carbon dioxide in 1-butyl-3-methylimidazolium acetate (Bmim Ac) has been studied by Raman spectroscopy and DFT calculations. It is shown that the solubility results from the existence of two distinct solvation regimes. In the first one (CO(2) mole fraction ≤ 0.35), the usual Fermi dyad is not observed, a fact never reported before for binary mixtures with organic liquids or ionic liquids (IL). Strong experimental evidence complemented by effective DFT modeling shows that this regime is dominated by a chemical reaction leading to the carboxylation of the imidazolium ring accompanied by acetic acid formation. The reactive scheme proposed involves two concerted mechanisms, which are a proton exchange process between the imidazolium cation and the acetate anion and the carboxylation process itself initiated from the formation of "transient" CO(2)-1-butyl-3-methylimidazole 2-ylidene carbene species. In that sense, CO(2) triggers the carboxylation reaction. Moreover, this dynamic picture circumvents consideration of a long-lived carbene formation in dense phase. The second regime is characterized by the detection of the CO(2) Fermi dyad showing that the carboxylation reaction has been strongly moderated. This finding has been interpreted as due to the interaction of the acetic acid molecules with the COO group of acetate anions involved in monodentate forms with the cation. The observation of the Fermi doublet allows us to infer that CO(2) essentially preserves its linear geometry and that the nature and strength of the interactions with its environment should be comparable to those existing in organic liquids and other IL as well. These results have been supported by DFT calculations showing that the CO(2) molecule interacts with energetically equivalent coexisting structures and that its geometry departs only slightly from the linearity. Finally, we find that the CO(2) solvation in Bmim Ac and 1-butyl-3-methylimidazolium trifluoroacetate (Bmim TFA) cannot be straightforwardly compared neither in the first regime due to the existence of a chemical reaction nor in the second regime because CO(2) interacts with a variety of environments not only consisting of ions pairs like in Bmim TFA but also with carboxylate and acetic acid molecule.

The interaction between water and hexafluorobenzene is discussed in light of an ab initio study. It is found that a dimer is formed with a binding energy of about −2 kcal/mol, with a geometry such that the oxygen of water is above the hexafluorobenzene molecule, with both hydrogen atoms pointing out of the ring, and the water C 2 axis is collinear with the main symmetry axis of the aromatic compound. In addition, the intermolecular potential surface (IPS) has been fully characterized. It comes out that the rotational motion of the water molecule around its C 2 axis is nearly “free”, whereas the two other bending motions are found to be strongly hindered. These results are discussed in comparison with those reported in the literature for the water−benzene dimer, in which a weak eπ−H hydrogen bond interaction has been reported. We argue that in the water−hexafluorobenzene system the oxygen atom acts as a Lewis base and that the aromatic ring plays the role of a Lewis acid, owing to the withdrawing effect of the fluorine atoms on the π-electronic distribution of the cycle. These results have been used to rationalize the far-infrared experiments that we have previously reported on water diluted in organic solvents. We emphasize that the viewpoint of the isolated dimer provided by our ab initio study could be transposed to the liquid state if the observation time of the technique is shorter than the lifetime of the dimer. It turns out that mid-infrared spectroscopy is more adapted, than far-infrared absorption, to put in evidence the existence of very labile dimers in the liquid phase, taking into account its shorter time window of observation.