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

摘要

The proton and fluorine chemical shifts of n-hexane, perfluoro-n-hexane, and 1,1-dihydroperfluorooctylpro-pionate 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.