Nuclear magnetic shielding of fluorine in the fluorides of the elements

Abstract

Nuclear magnetic shielding parameters for the (spin-paired) binary fluoride molecules and ions vary periodically with the atomic number of the central atom. The diamagnetic term σ_d has been calculated by Flygare's method, and experimental values of σ_p for 17 binary fluorides have been obtained from spin-rotation interaction constants (with Ramsey's and Flygare's equations) and from the shielding anisotropy for some linear molecules. These values of σ_p agree with those obtained by substracting σ_d(calc.) from the resultant shielding referred to an absolute scale, and σ_p has been obtained in this way for the remaining molecular fluorides and complex ions. The term σ_d increases periodically with the number of electrons in the molecule, but the resultant shielding follows the larger and more irregular variations in σ_p; |σ_p^F| increases sharply across the Row of the central atom for the typical elements and also for the transition metals with empty d(t_2g) orbitals. Fluorine is highly shielded, however, in the d⁶ hexafluoride molecules and ions, possibly because of circulations of the π*→σ* type which contribute positive terms to σ_p in CIF. The increase in |σ_p^F| across the Row of the central atom is ascribed to increasing imbalance of the fluorine 2p electrons with increasing covalency, to the increase in <r^–3>_2p, and to the tendency of (ΔE)^–1 to increase. Down the Group of the central atom |σ_p^F| tends to increase with (ΔE)^–1, against the tendency of <r^–3>_2p to decrease (for the typical elements), although there are important anomalies which may show the influence of the radius term. Near the bottom of the later Groups the increase in |σ_p^F| is slowed or reversed. Periodic correlations for other nuclei such as ¹³C show corresponding patterns of variation in σ_p and these are evident also in ¹H shielding. The periodic correlation shows the basis of the familiar relation between fluorine shielding and electronegativity of the central atom and of the important exceptions to this relation.