Journal of Supercritical Fluids, Vol.40, No.1, 20-26, 2007
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 beta-diketones (acetylacetone, trifluoroacetylacetone, and hexafluoroacetylacetone) and uranyl beta-diketonato complex UO2(hfacac)(2)DMSO] (hfacac: hexafluoroacetylacetone, DMSO: dimethyl sulfoxide). As a result, it was found that the Raman bands of SC-CO2 containing beta-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(hfaca)(2)DMSO. From these results, it was suggested that the Lewis acid-Lewis base (LA-LB) interactions between carbonyl oxygens of beta-diketones and CO2 carbon., and the hydrogen bondings between -OH of beta-diketones and CO2 oxygen are formed in SC-CO2 containing beta-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 (vi) of Raman bands of CO2 correlate with the donor number (DN) of compounds, i.e., v(i) = 1281.24-0.0109 DN at 25.0 MPa and 40 degrees C. (C) 2006 Elsevier B.V. All rights reserved.
Keywords:supercritical CO2;solute-solvent interaction;CO2-philicity;beta-diketones;beta-diketonato complex