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The infrared (IR) vibrational spectra of acetaminophen (N(4-hydroxyphenyl) acetamide or paracetamol) complexes formed with ethanol and acetone is investigated. It is argued that the exchange-correlation contribution taken into account in density functional theory calculations is likely significant in determining the main IR spectral features of acetaminophen complexes formed with acetone or involving hydrogen-bonded as with ethanol.

The 1:1 electron donor-acceptor complexes of CO2 formed with alcohols, methanol and ethanol, and with acetone in context of ab initio calculations are examined. The division of the v(sub 2) bending mode of CO2 and v(sub s)(OH) stretching mode of alcohols or the anti symmetric v(sub a)(CCC) and symmetric v(sub s)(CO) stretching modes of acetone are highlighted.

A study is conducted to evaluate the influence of the solute-solvent interactions on the infrared spectra of water diluted in liquid CCl(sub 4) and supercritical xenon. The findings indicate that the reorientational dynamics of water are more perturbed than those in SC xenon fluid, due to the weakly anisotropic character of the water-CCl(sub 4) interactions.

A study was conducted to analyze the interaction between water and hexafluorobenzene. The premise that the oxygen atom functions as a Lewis base in the water-hexafluorobenzene system was examined. Experimental results indicated that the dimer is formed with a binding energy of about -2 kcal/mol. Findings also showed that the rotational motion of the water molecule around the C2 axis is nearly free.

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. [ABSTRACT FROM AUTHOR]

The polarized and depolarized Raman spectra of CO2 have been measured as a function of CO2 concentration (0.02–0.7 molar fractions) in the dense phase of the binary mixtures obtained by introducing under pressure (from 0.2 up to 6.0 MPa) supercritical carbon dioxide (at 313 K) in liquid benzene. Four main experimental features are observed. A new weak polarized band centered at approximately 660 cm-1 has been detected in the region of the Raman inactive ν2 bending mode of carbon dioxide. The analysis of the polarized band shapes of the Fermi dyad shows that CO2 molecules probe two environments. In one of them carbon dioxide interacts “specifically” with benzene molecules, whereas in the other it interacts “nonspecifically” with its neighbors. The analysis of the depolarized Fermi dyad profiles shows that the rotational dynamics of CO2 specifically interacting with benzene is strongly hindered. Finally, a new weak polarized band has been detected between the two components of the dyad. These observations rationalized at the light of ab initio calculations show that CO2-benzene transient complexes are formed. It is argued that ab initio predictions, limited here to a pair of molecules, are still valid in dense phase because the elementary act of formation of the transient complex can be probed on the observation time and spatial range of vibrational Raman spectroscopy. [ABSTRACT FROM AUTHOR]

The polarized and depolarized Raman profiles of supercritical CO2 have been measured in the region of the ν2 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≤ρ*=ρ/ρC<2.04 (ρC∼467.6 kg m-3, ρC is the critical density). The spectral features associated with the ν2 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 CO2 molecule probed two kinds of environment in its first shell of neighbors independent of local density enhancement phenomenon. The first one involves interactions of CO2 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 CO2. [ABSTRACT FROM AUTHOR]

The near and midinfrared spectra of 1-octanol (and 2-octanol) have been measured along the liquid-gas coexistence curve from room temperature up to the critical point and in the supercritical domain along the isotherm T=385 °C (and T=365 °C) above the critical point of both 1-octanol and 2-octanol for pressure ranging from 0.5 up to 15 MPa. The density values of SC 1- and 2-octanol have been estimated by analysing the near infrared (NIR) spectra in the 3va(CH) region. A quantitative analysis of the absorption band associated with the OH stretching vibration [v(OH)] and its first and second overtones [2v(OH) and 3v(OH)] was carried out in order to estimate the percentage of “free” OH groups in both alcohols in the whole thermodynamic domain investigated here. Very consistent results have been obtained from the independent analysis of these three different absorption bands which gave us a good confidence in the degree of hydrogen bonding reported here for 1- and 2-octanol. Thus, the percentage of free OH groups which is around 5% in liquid 1-octanol under ambient conditions strongly increase up to 70%-80% at a temperature of about 340 °C. Then, in the supercritical domain, upon a decrease of the density from 0.4 to 0.1 g cm-3, the fraction of free hydroxyl groups is nearly constant presenting a plateaulike regime around 80%. As the density decreases again, this plateau regime is followed by a further increase of Xnb which reaches a value of 96% for the system in the gaseous phase (0.01 g cm-3; P=0.45 MPa). Finally, it comes out from this study that the percentage of free OH groups is always greater in 2-octanol than in 1-octanol at the same density. [ABSTRACT FROM AUTHOR]

Raman scattering combined with near- and midinfrared absorption spectroscopies was used to investigate the evolution of the local order in the water rich phase of water-CO2 mixtures under isobaric heating (T=40–360 °C, P=250 bars). The quantitative analysis of the spectra shows that tetramers and larger oligomers are the main constituents of water at moderate temperatures below 80 °C. As the temperature increases, the dimer and trimer concentrations considerably increase at the expense of larger oligomers. Finally, water dimers are predominant at the highest temperature investigated close to the temperature of total miscibility of the mixture (T=366 °C, P=250 bars). This result is consistent with our previous investigation [R. Oparin T. Tassaing, Y. Danten, and M. Besnard, J. Chem. Phys. 120, 10691 (2004)] on water dissolved in the CO2 rich phase where we found that close to the temperature of total miscibility water also exists mainly under dimeric form. The current study combined with that mentioned above provides a model investigation of the evolution of the state of aggregation of water molecules in binary mixture involving a hydrophobic solvent in a wide range of temperature. [ABSTRACT FROM AUTHOR]

Far-infrared (FIR) and mid-infrared (MIR) profiles of D2O infinitely dilute in supercritical CO2 have been studied using molecular-dynamics simulations. For this purpose, we have proposed an intermolecular potential model taking implicitly into account electron donor-acceptor (EDA) interactions between water and CO2 evaluated from ab initio calculations of the intermolecular potential-energy surface (IPS). Interaction-induced dipole mechanisms have been also taken into account in addition to the water permanent dipole to evaluate the simulated FIR profiles of water and CO2 polarizable molecules. They were found to play a minor role in the genesis of the FIR profiles of water/CO2 under supercritical conditions. The analysis of the reorientational dynamics of D2O shows that the rotational dynamics of water is weakly anisotropic due to the EDA interactions which affect more specifically the reorientational motions of the C2 symmetry axis of solute. These results have been used to assess the contribution of the vibrational relaxation in the experimental mid-infrared profiles associated with the ν1 symmetric and ν3 antisymmetric stretching and ν2 bending modes of D2O. It was found that the rotational dynamics mainly contribute to the broadening of the infrared (IR) profiles. Nevertheless, the vibrational processes play a role in the frequency shifts of the band centers and the relative intensity enhancements of the ν1 and ν3 modes of D2O. In particular, the EDA interactions between water and CO2 lead to the appearance of a well-defined IR band of the ν1 mode of D2O. Finally, a comparison with another model taking only into account dipole-quadrupole electrostatic interactions between water and CO2 molecules clearly reveals that EDA interactions have to be considered to reproduce both MIR and FIR measurements. From this point of view CO2 can be classified on a hydrophilic solvent scale based upon the solubility criterion as an intermediate solvent between “inert” xenon and carbon tetrachloride. [ABSTRACT FROM AUTHOR]

We have investigated the state of aggregation in supercritical tert-butanol (T=523 K,0.05<ρ<0.4 g cm-3) by means of vibrational spectroscopies (infrared and Raman) and molecular-dynamics (MD) simulations. A quantitative band shape analysis of the spectra associated with the OH stretching mode of tert-butanol has been done using activities computed by ab initio calculations on small clusters. This allows us to determine the degree of hydrogen bonding and populations of oligomers. These latter quantities have been derived from MD simulations and very consistent results are found with experiments. These results show that hydrogen bond still exist in supercritical tert-butanol and that the fluid mainly consists of oligomers smaller than tetramers. [ABSTRACT FROM AUTHOR]