Y. Danten

CO2 in 1-Butyl-3-methylimidazolium Acetate. 2. NMR Investigation of Chemical Reactions

The solvation of CO(2) in 1-butyl-3-methylimidazolium acetate (Bmim Ac) has been investigated by (1)H, (13)C, and (15)N NMR spectroscopy at low CO(2) molar fraction (mf) (x(CO(2)) ca. 0.27) corresponding to the reactive regime described in part 1 of this study. It is shown that a carboxylation reaction occurs between CO(2) and Bmim Ac, leading to the formation of a non-negligible amount (~16%) of 1-butyl-3-methylimidazolium-2-carboxylate. It is also found that acetic acid molecules are produced during this reaction and tend to form with elapsed time stable cyclic dimers existing in pure acid. A further series of experiments has been dedicated to characterize the influence of water traces on the carboxylation reaction. It is found that water, even at high ratio (0.15 mf), does not hamper the formation of the carboxylate species but lead to the formation of byproduct involving CO(2). The evolution with temperature of the resonance lines associated with the products of the reactions confirms that they have a different origin. The main byproduct has been assigned to bicarbonate. All these results confirm the existence of a reactive regime in the CO(2)-Bmim Ac system but different from that reported in the literature on the formation of a reversible molecular complex possibly accompanied by a minor chemical reaction. Finally, the reactive scheme interpreting the carboxylation reaction and the formation of acetic acid proposed in the literature is discussed. We found that the triggering of the carboxylation reaction is necessarily connected with the introduction of carbon dioxide in the IL. We argue that a more refined scheme is still needed to understand in details the different steps of the chemical reaction in the dense phase.

Infrared spectroscopic study of hydrogen-bonding in water at high temperature and pressure

Abstract It is the purpose of this paper to present an infrared absorption spectroscopic study of the state of aggregation of water over a wide range of temperature (25–380°C) and pressure (1–250 bar) along the liquid-gas coexistence curve, and in the supercritical domain. The evolution of the spectral profiles asociated with the internal vibrational modes and with the librational motion has been investigated. In supercritical water, at T=380°C, low pressures (densities) in the range 25–50 bar (0.01–0.05 g.cm−3), only monomeric water is detected. Progressive increase of the pressure (density), from 50 to 250 bar (from 0.05 to 0.4 g.cm−3), shows clearly the appearance of water dimers and trimers. Finally, upon decreasing the temperature (250-25 °C) along the liquid-gas coexistence curve, one observes a continuous evolution of the shape of the infrared spectrum characteristic of the presence of oligomers of increasing size and for temperatures lower than 200°C, the progressive appearance of the hydrogen bond network.

Solubility of CO2 in 1-Butyl-3-methyl-imidazolium-trifluoro Acetate Ionic Liquid Studied by Raman Spectroscopy and DFT Investigations

The polarized and depolarized Raman spectra of 1-butyl-3-methyl-imidazolium-trifluoro acetate (Bmim TFA) ionic liquid and of the dense phase obtained after introduction of supercritical carbon dioxide (313K) under pressure (from 0.1 MPa up to 9 MPa) in the ionic liquid have been recorded. The spectrum of the pure ionic liquid has been assigned by comparison with the spectra of ionic liquids sharing the same cation and using literature data concerning the vibrational modes of the TFA anion. It was found that the spectra of the ionic liquid is almost unaffected by the CO(2) dilution. The only noticeable perturbation concerns a weak enhancement of the mode assigned here to the symmetric stretch vibration of the COO group of the TFA anion. The band shape analysis of the ν(CC) band in pure Bmim TFA shows that the carboxylate groups probe a variety of environments which are almost not affected by the dilution in carbon dioxide. The analysis of the Fermi dyad of carbon dioxide shows that this molecule is perturbed upon dilution in the ionic liquid. The spectra suggest the presence of carbon dioxide in two different environments. In the first one, carbon dioxide molecules interact with themselves, whereas in the second environment, this molecule interacts with the COO group of the TFA anion. This is supported by B3LYP-DFT calculations aimed at assessing the interaction between an ion pair dimer and a carbon dioxide molecule. It is shown that dissolved CO(2) molecules preferentially interact with the TFA anion through a weak charge transfer interaction taking place between the carbon atom of CO(2) (acting as a Lewis acid) and a oxygen atom of the COO group of TFA (as a Lewis base). The results show that Bmim TFA is able to accommodate a large amount of carbon dioxide without having its short-range local structure significantly perturbed. Most CO(2) is hosted in the voids existing among the ion pairs, while some also weakly interact with the anion. It is finally argued that the evolution of the local organization of the IL upon carbon dioxide dilution presents similarities with the microsegregation phenomena reported for IL upon increasing the alkyl chains lengths.

Imidazolium-based ionic liquids: quantitative aspects in the far-infrared region.

The optical constants of some imidazolium-based ionic liquids (ILs) are determined in the mid- and far-infrared regions by combining polarized attenuated total reflection (ATR) and transmittance spectra. The internal vibrations of the cations and anions and the interionic vibrations can thus be quantitatively evaluated. A comparison of the far-IR spectral response of several imidazolium derivatives associated with the (CF(3)SO(2))(2)N(-) anion shows that methylation of the more acidic C((2))H imidazolium group does not change the far-IR intensity and hence that the CH...anion hydrogen bonds play a negligible role compared with electrostatic interactions. The calculated spectra of ion-pair dimers reproduce the far-IR density of states better than those of simple ion pairs.

Vibrational spectroscopic studies on the state of aggregation of water in carbon tetrachloride, in dioxane and in the mixed solvents

Abstract A detailed infrared study of the vibrational modes of water in dioxane, carbon tetrachloride and a mixed solvent as a function of concentration is reported. H 2 O/D 2 O mixtures have been used in order to isolate the uncoupled v(OH) and v(OD) modes. The data have been interpreted in terms of the state of aggregation of water molecules in a hydrophobic solvent and in a solvent to which hydrogen bonding can occur. It is shown that at very low concentrations ( x H 2 O = 5×10 −4 ) water exists in the monomeric form and can rotate rather freely. However, water is strongly self aggregated in a hydrophobic organic solvent for concentrations greater than x H 2 O = 5×10 −3 (ie if there are more than 5 water molecules in a 1000 solvent molecule ‘bath’). Evidence of distinct (dioxane) m (water) n complexes is found in dioxane solution and in the mixed solvent. However, as the water concentration is increased it is clear that a water/solvent ‘network’ is formed. The aggregation processes involving water depend critically, however, on the hydrophobicity of the solvent mixture.

A far infrared study of water diluted in hydrophobic solvents

The far infrared (FIR) absorption spectra of water diluted in various hydrophobic solvents (cyclohexane, n-heptane, carbon tetrachloride, benzene and its fluorinated derivatives) are reported. It is shown that the spectra are due essentially to the rotational motion of monomeric water in a solvent cage, the molecule behaving like a quasi free rotor when dissolved in cyclohexane and n-heptane, a slightly hindered rotor in carbon tetrachloride and a highly perturbed rotor in benzene. In the latter case our findings corroborate recent experimental and theoretical analyses concerning the specificity shown by the water-benzene ‘complex’. We also show that the FIR absorption spectrum is particularly sensitive to the presence of associated water in these solutions. Because of the very low solubility of water in the solvents investigated it is found that the presence of associated water is in fact the signature of incomplete equilibration of the mixture. Well equilibrated solutions show no evidence of association.

Transient dimer formation in supercritical carbon dioxide as seen from Raman scattering.

The polarized and depolarized Raman profiles of supercritical CO(2) have been measured in the region of the nu(2) bending mode (forbidden transition at about 668 cm(-1)) and for the Fermi dyad (1285 and 1388 cm(-1)) along the isotherms 307, 309, 313, and 323 K in a reduced density domain 0.04<rho*=rhorho(C)<2.04 (rho(C) approximately 467.6 kg m(-3), rho(C) is the critical density). The spectral features associated with the nu(2) mode (degeneracy removal of the mode and Raman intensity activation) are found to be due to the formation of transient complexes. This is supported by the spectral signatures predicted for parallel slipped dimer and trimers (cyclic and noncyclic) from ab initio calculations taking into account the frequency anharmonicity. The band-shape analysis of the Fermi doublet (observed in the spectral range of 1260-1400 cm(-1)) shows that on the subpicosecond time scale of the Raman spectroscopy, a tagged CO(2) molecule probed two kinds of environment in its first shell of neighbors independent of local density enhancement phenomenon. The first one involves interactions of CO(2) with surrounding molecules in the first shell whereas the latter is associated with a transient dimer formation. Finally, a broad band observed between the Fermi dyad (at about 1335 cm(-1)) is assessed from symmetry considerations and from its depolarization ratio as a further evidence of transient complex formation in supercritical CO(2).

A vibrational spectroscopic study of structure evolution of water dissolved in supercritical carbon dioxide under isobaric heating

A combination of Raman scattering spectroscopy and infrared absorption was applied to investigate the structural evolution of water dissolved in supercritical carbon dioxide under isobaric heating (T=40-340 degrees C, P=250 bar). Quantitative analysis of experimental spectra allowed us to determine that at relatively moderate temperatures water dissolved in CO(2)-rich phase exists only under monomeric form (solitary water surrounding by CO(2) molecules), but hydrogen-bonded species, namely, dimers, begin to appear upon heating. At the same time, the ratio of dimers to monomers concentration increases with further temperature increase and at temperatures close to the temperature of total miscibility of the mixture (T=366 degrees C, P=250 bar), water dimers only are present in the CO(2)-rich phase.

Spectroscopic studies of vibrational relaxation and chemical exchange broadening in hydrogen-bonded systems. III. Equilibrium processes in the pyridine/water system

Raman (isotropic) scattering for the v 1 and v 12 modes of pyridine in binary solutions with water have been studied over the full concentration range. Band intensity data, based on a fit to three Lorentzian bands, are consistent, at least up to x W ≅ 0·6, with an equilibrium model which includes three different pyridine/water complexes Pyr2·W, Pyr·W and Pyr·W2, and which includes dimeric and trimeric water aggregates. The likely uniqueness of this interpretation is discussed in the light of recent mass spectrometric and infrared evidence that, especially at higher water concentrations, the binary mixture is more likely to contain, on the time scale of the experiment, a networked distribution of hydrogen-bonded interactions rather than a set of discrete ‘complexes’.

A far infrared study of benzene-fluorinated benzene binary mixtures

Abstract The experimental FIR spectra of liquid equimolar binary mixtures formed with benzene, 1,3,5-trifluorobenzene and hexafluorobenzene have been compared with the spectra calculated as a weighted sum of the corresponding spectra of the neat liquids. In contrast to the other binary mixtures, for C 6 H 6 /C 6 F 6 a marked deviation from the additivity is observed. These results are interpreted as providing evidence for the existence of a different and stronger local ordering which might be ascribed as the “signature” of a very short-lived “complex”.