Positional and Geometric Characterization of Olefinic Double Bonds by Fluorine Magnetic Resonance Spectrometry

Conversion of n-alkenes to hexafluoroacetone ketals has allowed the characterization of the double bond position and geometry using ¹⁹F NMR. Geometry can be determined by the chemical shift, with a 0.775 ppm separation between the els and trans isomers. Double bond position can be determined by the value of Δv _as for the A₃B₃ pattern, which decreases as the bond moves to the center of the molecule. A method by which Δv _as may be estimated without the use of computer simulation is discussed. Application of these findings to the quantitative determination of geometric mixtures of linear alkenes is examined. Prior separation is required for analysis of positional mixtures but not for geometric mixtures.