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 CO2 density differentially influences H-1 and F-19 chemical shifts, which, in turn, suggest specific solute-solvent interactions between CO2 and fluorinated compounds. The density-dependent proton chemical shifts are exclusively governed by changes in the bulk susceptibility of CO2; these shifts exhibit only a slight temperature dependence. By contrast, the F-19 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 CO2 density and temperature. Furthermore, the F-19 NMR data also show a distinct site specificity which is qualitatively explained in terms of the surface accessibility of fluorine atoms in the various CF2(CF3) units in fluorocarbons.