Site-selective solvation in supercritical CO2 observed by Raman spectroscopy: phenyl group leads to greater attractive energy than chloro group.

Vibrational Raman spectra of the C=C stretching modes of cis-stilbene and cis-1,2-dichloroethylene (C(2)H(2)Cl(2)) were measured in supercritical CO(2) in a density range of 0.08 < ρ(r) = ρ/ρ(c) < 1.5 at an isotherm of T(r) = T/T(c) = 1.02. As the fluid density increased, the peak frequencies of cis-stilbene and cis-C(2)H(2)Cl(2) shifted toward the low-energy side. The shifted frequencies of cis-stilbene were consistently greater than those of cis-C(2)H(2)Cl(2) in all density regions, by a factor of 4. By analyzing these density dependencies using the perturbed hard-sphere theory, the shifted frequencies were decomposed into attractive and repulsive components. By quantifying these components as a function of fluid density, we investigated how each solute is solvated in supercritical CO(2). The results indicate that the attractive energy between cis-stilbene and CO(2) is twice that between cis-C(2)H(2)Cl(2) and CO(2). A local density augmentation around the solute molecule was not observed in the cis-C(2)H(2)Cl(2)/CO(2) system, but it was observed in the cis-stilbene/CO(2) system because of site-selective solvation around the phenyl group of cis-stilbene. To the best of our knowledge, this is the first time that the site-selective solvation of a solute molecule has been observed using Raman spectral measurements of a solution system. Based on theoretical calculations and Raman spectral measurements of cis-stilbene in the supercritical fluid of dipolar CHF(3), it is concluded that a driving force for site-selective solvation is the dispersion force.