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19 F broad line nuclear magnetic resonance study Barr, Matthew Ronald


A general broad line nuclear magnetic resonance study was made of the ¹⁹F spectra of WF₆ and the adducts IF₇ • AsF₅ and SF₄ • AsF₅ to determine the temperature dependence of the spectra, interpret the line shapes with respect to isotropic and anisotropic chemical shifts and identify non-equivalent fluorine sites in the compounds. The temperature dependence of the second moment at 30 MHz indicated that rigid lattice conditions probably existed at 77°K for WF₆ and IF₇ • AsF₅ but not for SF₄ • AsF₅. The dependence indicated an nmr transition in the vicinity of 200°K for the first two compounds and one commencing below 77°K for the third. From the second moments in the vicinity of the transitions, activation energies mere determined for the average motions involved. The field dependence of the second moments of the compounds was examined, where possible, at 2, 16, 30, 40, 56.4, and 94.1 MHz at 77° and 295°K. The compounds' spectra were resolved, with varying degrees of success, into components. For WF₆ an approximate resolution could be made into two components corresponding to the four equatorial and two axial fluorines in the distorted octahedron at 77°K. The two adducts could both be resolved, especially at 295°K or above, into components which supported the ionic formulations IF⁺₆ AsF⁻₆ and SF⁺₃ AsF⁻₆. Non-equivalent fluorine sites within individual ions could not be detected. From the observed and estimated second moments of the resolved components above and below the transitions, the probable reorientations occuring above the transitions were suggested. The rigid lattice theoretical second moment calculations enabled suggestions to be made for the crystal structures of WF₅ and SF⁺₃AsF⁻₆ and for the bond lengths in IF⁺₆AsF⁻₆ . For the first there had been confusion, at least here, about the space group, while the second has not yet been the subject of reported X-ray studies. Axial symmetry of the chemical shift tensors was assumed. Then, taking account of the relative shifts between the resolved components, averaqe values of the chemical shift anisotropies for each of WF₆ and IF⁺₆ AsF⁻₆ were determined from expressions relating the field squared dependence of the second moment to those quantities. The mean isotropic shifts of the total F spectra for each compound were measured where possible at each field at 77° and 295° K with respect to CF₃. COOH. From those the shifts of the resolved components were calculated relative to HF. Then from the isotropic shifts and the anisotropies, the principal values of the axially symmetric shift tensors were determined. The principal values enabled estimates to be made of I (ionic) and ℓ(double bond) characters, neglecting hybridization, in the M-F bonds of the hexafluoride groups. From these values a prediction was made for I and ℓ in the axial and equatorial bonds in PuF₆ .

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