1 9F NMR chemical shifts due to intermolecular interactions in F2C=CFX. A quantitative measure of the nuclear site effect

Abstract

In CF2=CFX (X=H, Cl, Br, I) there are three probe nuclei for studying the magnitudes of intermolecular effects on nuclear shielding. The values of σ1=limρ→0 (∂σ/∂ρ)T, the change in the nuclear shielding due to interactions between pairs of molecules, have been obtained from the resonance frequencies in medium to low density gas samples. The nuclear site effect gives the most exposed one of the three probe nuclei the largest magnitude of σ1, i.e., the F trans to X=Cl, Br, I or the F cis to X=H. The gas-to-liquid shifts show exactly the same ordering as the σ1 in the dilute gas. A model is described which relates a calculated site factor to the observed σ1 values. With this model, the observed density dependence of the NMR isotope shift in D2/HD is calculated. This model provides a mechanism for nonspecific solvent effects on the NMR isotope shift.