Transient complex formation in CO2-hexafluorobenzene mixtures: a combined raman and ab initio investigation

The polarized and depolarized Raman spectra of CO2 in binary mixtures with hexafluorobenzene have been measured in the dense phase along the isotherm 313 K as a function of concentration (0.03-0.7 molar fraction in CO2) by varying the pressure (0.5-6.2 MPa). Experimental observations in the nu2 bending region, in the nu1-2nu2 Fermi resonance dyad, and in the spectral domain between the Fermi dyad peaks on CO2 are reported. These results are discussed in comparison with those obtained in previous studies on CO2-C6H6 and CO2-acetone mixtures. We conclude in agreement with previous investigations that CO2 molecules can probe two environments. In one of them carbon dioxide interacts "specifically" with hexafluorobenzene molecules to form a transient heterodimer, whereas in the other environment CO2 interacts "nonspecifically" with its neighbors. New ab initio calculations reported here allow rationalizing most of the experimental results. However, the observation of weak spectral features (bending mode and Fermi dyad regions) shows that a slight departure from the predicted structure (C6v symmetry) should exist in the dense phase. Finally, the greater solubility of CO2 in perfluorinated benzene versus perhydrogenated benzene has been discussed on the basis of this study in connection with thermodynamic measurements interpreted in the scaled particle theory framework.