Raman spectroscopy and ab initio investigations of transient complex formation in CO2-benzene mixtures

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 nu(2) 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.