Local density inhomogeneities detected by Raman scattering in supercritical hexafluorobenzene

Abstract: The influence of the local density inhomogeneities in supercritical hexafluorobenzene C 6 F 6 has been assessed using Raman spectroscopy. The polarized and depolarized profiles associated with the ν 1 (A 1g ) "breathing" mode of the molecule has been analyzed for the fluid in a wide density range (0.1 ≤ ρ* = ρ/ρ C ≤ 3), namely under isothermal conditions (T* = T/T C~ 1.11 and close to the critical isotherm T*~1.02). The evolution upon the density of the band center position of the isotropic profile along the near-critical isotherm showed an anomalous behavior, characterized by a plateau in the density range (0.6 ≤ ρ* = ρ/ρ C ≤ 1.3), which is not observed along the isotherm T* ~ 1.11. It has been interpreted as due to the existence of local density inhomogeneities and the density enhancement factor has been evaluated. The rotational dynamics of the main symmetry axis of the molecule is governed by a diffusional process. The rotational correlation time τ 2R exhibits an anomalous behavior (plateau regime) for both isotherms. These findings put in evidence the existence of local density inhomogeneities in a pure fluid and show that Raman spectroscopy is well adapted to investigate these phenomena

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 (nu(2), nu(3)) modes of CS2 and of the nu(1)-2 nu(2) 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. (C) 2013 AIP Publishing LLC

Investigating the potential of pyrazine dioxide based-compounds as organic electrodes for batteries

International audienceUnderstanding structure-property relationship in redox-active molecular species is of central importance in various fields, including many medicinal and chemical applications. The quest for performant organic electrodes in the context of energy storage calls for pioneering studies to develop new and possibly optimal materials. Beyond modifying the molecular design of the existing compounds through functionalization, expansion of the search enabling the advent of efficient new backbones can potentially lead to breakthroughs in this research area. The number of already identified families able to constitute negative organic electrodes is much lower than that of their positive counterparts, which calls for finding ways to bridge this gap. To expand the dataset of known predicted redox potentials and in view of reaching an educated guess about the abilities of some eventual new redox active electrodes, we examined the properties of pyrazine N,N'-dioxide (PZDO) and its fully methylated functionalized derivative (TeMePzDO). The aspects and mechanisms driving the various features characteristic of these compounds were unraveled through molecular and periodic DFT calculations combined with accurate electronic structure analysis. The predicted molecular redox/crystalline intercalation potentials lead to the classification of PZDO and TeMePzDO systems within the class of negative electrodes, with features that are significantly appealing compared to those of some existing systems with backbones suited for such kind of application

Why is the CO2-CS2 non-ideality larger than in CO2-CCl4? A Raman scattering study

The dense phases of the CO2-CCl4 (I) and CO2-CS2 (II) mixtures have been studied by Raman spectroscopy. Mixture I is found almost ideal and II strongly non-ideal. At high CS2 concentration in II, the local structure of CS2 is preserved suggesting a nano-segregation of the liquid phase without demixing whereas in CO2 concentrated mixtures a diversity of species are present. Thermodynamical considerations together with spectroscopic results show that a subtle interplay between attractive and repulsive interactions leads to this non-ideal behaviour. The connection of these results with the liquid-liquid demixing reported for CO2-CS2 at lower temperature is discussed. (C) 2013 Elsevier B. V. All rights reserved