The mean bond displacements in O=CF2 and their effect on 19F nuclear magnetic shielding

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Abstract
The mass and temperature dependence of the mean bond displacements in carbonyl fluoride have been determined using a modified Urey–Bradley field, by solving the set of coupled equations relating mean bond displacements 〈ΔRCF〉 and 〈ΔrCO〉 to mean square-vibrational amplitudes 〈(ΔRCF)2〉,〈ΔrΔR〉, etc. To these vibrational terms the rotational (centrifugal distortion) contributions are added. The results are applied to the interpretation of the temperature dependence of the 19F nuclear shielding in O=CF2 at the zero-pressure limit. By fitting the observed [σ0(T)−σ0(300)] for T=250–350 K an empirical value of (∂σ/∂ΔrCF)e=−1146 ppm Å−1 is obtained. This derivative is used to calculate the NMR isotope shifts from the mass dependent 〈ΔRCF〉. The calculated value, [σ0(16O12CF2) −σ0(16O13CF2)] =−0.096 ppm, is in good agreement with our measured value of −0.117 ppm. From the fitting to the temperature dependence of σ0 we also get σe−σ0 (300 K)=7 ppm. This and the absolute shielding σ0 (300 K)=221.6 ppm yields an estimate for 19F shielding at the equilibrium configuration, σe≂229 ppm. The errors which are associated with these values are discussed.