DFT Study of the Reaction Mechanisms of Carbon Dioxide and its Isoelectronic Molecules CS2 and OCS Dissolved in Pyrrolidinium and Imidazolium Acetate Ionic Liquids.

The reaction mechanisms of CO2 and its isoelectronic molecules OCS and CS2 dissolved in N-butyl-N-methylpyrrolidinium acetate and in 1-butyl-3-methylimidazolium acetate were investigated by DFT calculations in "gas phase". The analysis of predicted multistep pathways allowed calculating energies of reaction and energy barriers of the processes. The major role played by the acetate anion in the degradation of the solutes CS2 and OCS as well as in the capture of OCS and CO2 by the imidazolium ring is highlighted. In both ionic liquids, this anion governs the conversion of CS2 into OCS and of OCS into CO2 through interatomic S-O exchanges between the anion and the solutes with formation of thioacetate anions. In imidazolium acetate, the selective capture of CS2 and OCS by the imidazolium ring competes with the S-O exchanges. From the calculated values of the energy barriers a basicity scale of the anions is proposed. The (13)C NMR chemical shifts of the predicted adducts were calculated and agree well with the experimental observations. It is argued that the scenario issued from the calculated pathways is shown qualitatively to be independent from the functionals and basis set used, constitute a valuable tool in the understanding of chemical reactions taking place in liquid phase.

Assessing the non-ideality of the CO2-CS2 system at molecular level: a Raman scattering study.

The dense phase of CO2-CS2 mixtures has been analysed by Raman spectroscopy as a function of the CO2 concentration (0.02-0.95 mole fractions) by varying the pressure (0.5 MPa up to 7.7 MPa) at constant temperature (313 K). The polarised and depolarised spectra of the induced (ν2, ν3) modes of CS2 and of the ν1-2ν2 Fermi resonance dyad of both CO2 and CS2 have been measured. Upon dilution with CO2, the evolution of the spectroscopic observables of all these modes displays a "plateau-like" region in the CO2 mole fraction 0.3-0.7 never previously observed in CO2-organic liquids mixtures. The bandshape and intensity of the induced modes of CS2 are similar to those of pure CS2 up to equimolar concentration, after which variations occur. The preservation of the local ordering from pure CS2 to equimolar concentration together with the non-linear evolution of the spectroscopic observables allows inferring that two solvation regimes exist with a transition occurring in the plateau domain. In the first regime, corresponding to CS2 concentrated mixtures, the liquid phase is segregated with dominant CS2 clusters, whereas, in the second one, CO2 monomers and dimers and CO2-CS2 hetero-dimers coexist dynamically on a picosecond time-scale. It is demonstrated that the subtle interplay between attractive and repulsive interactions which provides a molecular interpretation of the non-ideality of the CO2-CS2 mixture allows rationalizing the volume expansion and the existence of the plateau-like region observed in the pressure-composition diagram previously ascribed to the proximity of an upper critical solution temperature at lower temperatures.

Interaction of water highly diluted in 1-alkyl-3-methyl imidazolium ionic liquids with the PF6(-) and BF4(-) anions.

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.