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

What is the state of aggregation of ethanol molecules in ethanol-supercritical carbon dioxide mixtures? An FTIR investigation in the full molar fraction range

The evolution of the degree of hydrogen bonding of ethanol molecules in scCO2–ethanol mixtures for different molar fraction (from 0.5 to 100% in ethanol) in the temperature range 40–200 °C and at two different constant pressures P = 15 and 20 MPa is reported in this paper. For a given pressure, we observe a strong dependence of the degree of hydrogen bonding as a function of temperature and ethanol molar fraction. We emphasize that a detailed knowledge of the degree of hydrogen bonding of ethanol molecules in these binary systems is important for the understanding and the further development of the supercritical fluid technology

On the cluster composition of supercritical water combining molecular modeling and vibrational spectroscopic data.

International audienceThe present study is aimed at a detailed anal. of supercrit. H2O structure based on the combination of exptl. vibrational spectra as well as mol. modeling calcns. of isolated H2O clusters. The authors propose an equil. cluster compn. model where supercrit. H2O is considered as an ideal mixt. of small H2O clusters (n = 1-3) at the chem. equil. and the vibrational spectra are expected to result from the superposition of the spectra of the individual clusters, Thus, it was possible to ext. from the decompn. of the midinfrared spectra the evolution of the partition of clusters in supercrit. H2O as a function of d. The cluster compn. predicted by this model is quant. consistent with the near IR and Raman spectra of supercrit. H2O analyzed using the same procedure. The authors emphasize that such methodol. could be applied to det. the portion of cluster in H2O in a wider thermodn. range as well as in more complex aq. supercrit. solns. (c) 2010 American Institute of Physics. [on SciFinder(R)

The partial pair correlation functions of dense supercritical water

Neutron diffraction measurements of heavy water and of two isotopic ${\rm H_2 O/D_2 O}$ mixtures at supercritical state ($T=380\ {}^\circ{\rm C}$ and $\rho_{\rm D_2O}=0.73\,{\rm g/cm}^3$) are presented. In combining the set of neutron diffraction data with previous X-rays measurements of Yamanaka et al. (J. Chem. Phys., 101 (1994) 9830), it has been possible by using a Monte Carlo method to reach the partial pair correlation functions $g_{\rm OH}(r)$, $g_{\rm HH}(r)$ and $g_{\rm OO}(r)$. The results are compared with molecular-dynamics simulations using the SPCE pair potential for water. These new results confirm that hydrogen bonding is still present in dense supercritical water

Calculation of IR frequencies and intensities in electrical and mechanical anharmonicity approximations: Application to small water clusters.

International audienceThe authors present a method for automatic computation of IR intensities using parallel variational multiple window CI wave functions (P_VMWC2 algorithm). Inclusion of both mech. and elec. anharmonic effects permits fundamental vibrational frequencies, including combinations and overtones, to be assigned. The authors use these developments to interpret the near-IR (NIR) and mid-IR (MIR) spectra of individual H2O clusters (H2O)n (n = 1-4). Cyclic and linear systems were studied to provide equiv. ref. theor. data to study the structure of H2O as a function of d. using NIR and MIR exptl. spectra. Various d. functional theory methods for generating the potential energy surface were compared to ref. results obtained at the CCSD(T) level. For cyclic clusters, the IR intensities and frequencies obtained using B1LYP/cc-pVTZ are in very good agreement with the available exptl. values and of the same orders of magnitude as the ref. theor. values. These data are completed by the vibrational study of linear systems. (c) 2010 American Institute of Physics. [on SciFinder(R)