Solvation Structures of cis- and trans-1,2-Dichloroethylene in Supercritical CO2 Investigated by Raman Spectroscopy and Attractive Energy Calculations

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Abstract
Vibrational Raman spectra of the C═C stretching modes of cis- and trans-1,2-dichloroethylene (C2H2Cl2) were measured in supercritical carbon dioxide (CO2). The spectra were collected at a fixed solute mole fraction by varying the fluid density by a factor of 20. As the density increased, the peak frequencies of the C═C stretching modes shifted toward the low-energy side at isotherms of reduced temperature, Tr = T/Tc = 1.02, 1.06, and 1.20. By analyzing these density dependences using the perturbed hard-sphere theory, we decomposed the shifts into attractive and repulsive components. The repulsive shifts of cis-C2H2Cl2 were almost equivalent to those of trans-C2H2Cl2. However, the attractive shifts of nonpolar trans-C2H2Cl2 were significantly greater than those of polar cis-C2H2Cl2 at all densities and temperatures. To evaluate the difference in the isomers, we calculated the attractive shifts of the C═C stretching modes of each isomer, composing of dispersion, dipole-induced-dipole, and dipole−quadrupole interactions between solute C2H2Cl2 and solvent CO2 molecules. These three interactions were quantified by considering molecular configurations and orientations, and solvation structures around the isomers were elucidated by 3D schematic diagrams. As a result, it was shown that the anisotropic solvation structure around trans-C2H2Cl2 was responsible for the larger attractive shifts in the supercritical CO2. The difference of solvation structures between the isomers was significant at Tr = 1.02 but became minor as the temperature increased to Tr = 1.20.

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