The temperature dependence of the 19F resonance in isolated F2C=CFX molecules

Abstract

The 19F nuclear magnetic shielding in a molecule of the type F2C=CFX in the zero-pressure limit is of particular interest since the shielding in three different electronic environments are averaged over the anharmonic motions described by a single force field. The temperature dependence of the nuclear shielding for the three 19F nuclei in the molecules F2C=CFX (X=H, Cl, Br, I) are reported here. There are systematic changes in the temperature coefficient of the nuclear shielding upon X substitution. For these molecules, the values of −(dσ0/dT)300 decrease in the order X=I, Br, Cl, F, H for the F nuclei which are gem and cis to X. On the other hand, for the F nucleus which is trans to X, the values of −(dσ0/dT)300 are very similar for X=I, Br, Cl, F, and all are greater than for X=H. Comparison of the coupling constants obtained in the gas phase with values in various solvents provide support for Barfield and Johnston’s theory of solvent effects on spin–spin coupling.