The mean bond displacements and the derivatives of 19F shielding in CF2=CFX and CF2=CH2

The mean bond displacements 〈Δr〉 for molecules CF2=CFX (X=H, F, Cl, Br, I) and CF2=CH2 have been calculated as a function of temperature using a Urey–Bradley force field augmented by stretching anharmonic terms and cubic nonbonded interactions. The temperature dependence of the 19F nuclear shielding in the isolated molecule limit is interpreted in terms of 〈ΔrCF〉T and an electronic factor (∂σF/∂ΔrCF)e which is found by fitting the experimental data with σ0(T)−σ0(300 K)=(∂σF/∂ΔrCF)e[〈ΔrCF〉T −〈ΔrCF〉300] . The derivatives for each of the three F sites in the CF2=CFX molecules vary systematically with the absolute shielding of the F site. The 19F nucleus gem to the X substituent exhibits the greatest change of shielding with bond displacement. For X=Cl, Br, I, the derivatives (∂σF/∂ΔrCF)e are (−1940, −2000, −1950 ppm Å−1) for the F nucleus trans to X; (−2190, −2290, −2300 ppm Å−1) for the F nucleus cis to X; and (−2550, −2890, −3160 ppm Å−1) for the F nucleus gem to X. In CF2=CF2 and CF2=CH2 the derivatives are −2080 and −990 ppm Å−1, respectively. For CF2=CFH they are −1250, −1190, and −2260 ppm Å−1 for the F nucleus trans, cis, and gem, respectively, to the H substituent.