Raman spectral shifts of CO2 as measure of CO2-philicity of solutes in supercritical carbon dioxide

We have examined the solute–solvent interactions by measuring Raman spectra of CO2 in neat supercritical CO2 (sc-CO2), and sc-CO2 containing β-diketones (acetylacetone, trifluoroacetylacetone, and hexafluoroacetylacetone) and uranyl β-diketonato complex [UO2(hfacac)2DMSO] (hfacac: hexafluoroacetylacetone, DMSO: dimethyl sulfoxide). As a result, it was found that the Raman bands of sc-CO2 containing β-diketones are shifted to lower wavenumber compared with those of neat sc-CO2 and that such a red-shift is not observed in sc-CO2 containing UO2(hfacac)2DMSO. From these results, it was suggested that the Lewis acid–Lewis base (LA–LB) interactions between carbonyl oxygens of β-diketones and CO2 carbon, and the hydrogen bondings between OH of β-diketones and CO2 oxygen are formed in sc-CO2 containing β-diketones, and that the shifts in Raman bonds of sc-CO2 can be used as the measure of CO2-philicity of solutes in sc-CO2. On the basis of such results, we measured Raman spectra of CO2 in sc-CO2 containing compounds with different donicity. As a result, we found out that the wavenumbers (νl) of Raman bands of CO2 correlate with the donor number (DN) of compounds, i.e., νl = 1281.24–0.0109 DN at 25.0 MPa and 40 °C.