Systematic trends in the variation of19F nuclear magnetic shielding with bond extension in halomethanes

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Abstract
A modified Urey-Bradley force field using the same set of quadratic constants for all halomethanes, augmented by Morse stretching and Lennard-Jones non-bonded anharmonic force constants, is used to calculate mean bond displacements in two series of halomethanes CF4, CF3Cl, CF3Br, CF3I and CF4, CF3Cl, CF2Cl2, CFCl3. The temperature dependence of the <Δr CF>T in these molecules is used in a one-parameter fit to the observed temperature dependence of the 19F shielding at the zero-pressure limit, [σ0(T) - σ0(300)]. The empirical derivatives (∂σF/∂Δr CF)e are then used to calculate the 13/12C-induced 19F isotope shifts at room temperature. The calculated isotope shifts are in satisfactory agreement with experimental values. The derivatives are: (∂σF/∂Δr CF)e = -1180, -1630, -1850, -1950, -2000, and -2400 ppm Å-1 respectively for CF4, CF3Cl, CF3Br, CF3I, CF2Cl2, and CFCl3. The CF3 X molecules exhibit a nearly linear plot of (∂σF/∂Δr CF)e vs. σe. For the series CF4-n Cl n the derivative varies linearly with n. For the series CF3 X, X = F, Cl, Br, I, the derivative of the isotropic shielding varies linearly with the component of the shielding tensor which is perpendicular to the XCF plane and that which is along the C-F bond direction.