1H, 13C, and 19F NMR Studies on Molecular Interactions of CO2 with β-Diketones and UO2(β-diketonato)2DMSO Complexes in Supercritical CO2

Abstract

The relationship between solubility of beta-diketones and uranyl beta-diketonato complexes (UO2(beta-diketonato)2DMSO) in supercritical carbon dioxide (scCO2) and their molecular interactions has been studied by measuring 1H, 13C, and 19F NMR chemical shifts at 40 degrees C in the pressure range of 7-25 MPa (density; 0.2 < rho (g cm(-3)) < 0.9). The 1H and 19F NMR spectra showed that beta-diketones exist mainly as enol form in scCO2 and that UO2(beta-diketonato)2DMSO in scCO2 have the same structures as those in nonaqueous solvents with low dielectric constants. It was found that scCO2 forms the van der Waals interactions with both proton and fluorine groups in beta-diketones and UO2(beta-diketonato)2DMSO and that such interactions of scCO2 with fluorine are much stronger than those with proton. However, the 13C NMR chemical shifts (13C-deltas) of scCO2 containing solutes such as beta-diketones showed a quite different tendency from 1H and 19F NMR chemical shifts. The 13C-deltas moved entirely to lower field direction in parallel with those of neat scCO2, whereas those of scCO2 containing UO2(beta-diketonato)2DMSO did not show such parallel shifts. The magnitude of parallel shifts (deltadeltac) of scCO2 was found to be dependent on the properties of solutes in scCO2 and to decrease in order of fluorinated beta-diketone > non-fluorinated beta-diketone > organic solvents having C=O groups and hydrogen-bonding OH groups > UO2(beta-diketonato)2DMSO. This order was quite consistent with the magnitude of solubility of corresponding compounds in scCO2. These results indicate that the differences of deltadeltac values are mainly attributable to the strength of solute-CO2 interactions through the Lewis acid-Lewis base (LA-LB) interactions and can be used as measure for evaluating the CO2-philicity of solutes in scCO2. It can be concluded that the solubility of UO2(beta-diketonato)2DMSO and beta-diketones in scCO2 is mainly controlled by the balance among the van der Waals interactions, LA-LB interactions, CH...O interactions between CH3 protons and CO2 oxygen atoms, and steric configuration between CO2 and the solutes.