Fluorocarbons Dissolved in Supercritical Carbon Dioxide. NMR Evidence for Specific Solute−Solvent Interactions

The proton and fluorine chemical shifts of n -hexane, perfluoro- n -hexane, and 1,1-dihydroperfluorooctylpropionate dissolved in supercritical carbon dioxide have been studied using high-pressure, high-resolution nuclear magnetic resonance. The CO 2 density differentially influences 1 H and 19 F chemical shifts, which, in turn, suggest specific solute−solvent interactions between CO 2 and fluorinated compounds. The density-dependent proton chemical shifts are exclusively governed by changes in the bulk susceptibility of CO 2; these shifts exhibit only a slight temperature dependence. By contrast, the 19 F signals show that an additional mechanism a van der Waals term contributes to the magnetic shielding, and this has to be taken into account to explain the experimental shifts as a function of CO 2 density and temperature. Furthermore, the 19 F NMR data also show a distinct site specificity which is qualitatively explained in terms of the surface accessibility of fluorine atoms in the various CF 2 (CF 3 ) units in fluorocarbons.