Dipolar Broadening of the NMR Spectrum of ¹⁹F in UF₄

Abstract

The influence of internuclear dipole-dipole and electron-nuclear dipolar interactions on the NMR spectrum shape was examined for UF,. It is shown that only the dipolar interaction of the l9F nuclei with the magnetic moments of the U4+ ions leads t o such a broad NMR spectrum in polyrrystalline UF, that there is no ground for separating it into several components corresponding to the seven nonequivalent fluorine nuclei in the UF, structure. A certain additional broadening of the experimental spectrum may be accounted for by anisotropic hyperfine interaction, due t o the transfer of the spin density from UZt to fluorine. On studying the magnetic susceptibility of UF, in the region of helium temperatures, Leask et al.[l] arrived a t the conclusion that the theoretical magnetic moment of the U4+ ion is essentially larger than the observed one either on account of indirect interaction or due t o the "spread" of unpaired electrons of the U4+ ions along the adjacent surroundings. The transfer of spin density t o fluorine atoms in UF, should lead to hyperfine interaction of fluorine nuclei with unpaired electrons which, in principle, may be detected by the NMR and ESR methods.Schulman and Jaccarino [a] noted that in the case of the 19F NMR spectrum of uranium tetrafluoride there is no isotropic shift which could be ascribed t o isotropic hyperfine interaction. However, later it was noted [3] that the absence of a spectrum shift simply indicates that the isotropic component of the hyperfine interaction is equal to zero, whereas the anisotropic component, leading t o a broadening of the spectrum and to no subsequent shift, may have a nonzero value. I n other words, this signifies that 2p states, and not 2s states, of fluorine are liable for mixing with the magnetic states of the U4+ ion. Hov ever, in order to discover the above-said anisotropic hyperfine interaction by means of the polycrystal NMR spectrum (UF4 monocrystals of sufficient size cannot yet be obtained), i t is essential to account for several factors which likewise lead t o a broadening of the spectrum.Pintar in his recent paper [4], however, gave a different interpretation for the l9F NMR spectra in UF,. Thus, he explains thc absence of spectrum shift by the presence of seven non-equivalent fluorine nuclei in the structure of uranium 28 *

Characterizing the solid hydrolysis product, UF4(H2O)2.5, generated from neat water reactions with UF4 at room temperature

“…Like Raman spectroscopy, few modern studies have been published on the NMR of UF4 or the related uranyl fluoride. 32,33 The lack of modern NMR data for UF4 may be due to interactions between 19 F and paramagnetic U 4+ ions, which lead to a broad, two-peak spectrum that cannot be separated into components corresponding to the multiple nonequivalent fluorine sites in UF4. However, NMR of 19 F in UF4 can still be informative for tracking chemical changes caused by reactions with water, particularly as U 4+ ions convert to diamagnetic U 6+ ions.…”

Exceptional uranium(VI)-nitride triple bond covalency from 15N nuclear magnetic resonance spectroscopy and quantum chemical analysis

“…The generally accepted chemical bonding picture is that for early actinides the bonding varies from ionic to polarised-covalent as a function of An-oxidation state and ligands, with this range sitting intermediate to the ionic lanthanides and usually much more covalent d transition metal complexes 5,7,8,10,11 . Probing actinide covalency is challenging, but in recent years progress has been made using experimental approaches, underpinned by quantum chemical calculations, including K-edge X-ray absorption near edge spectroscopy [12][13][14][15][16][17][18] , pulsed electron paramagnetic resonance spectroscopy 19 , and nuclear magnetic resonance (NMR) spectroscopy [20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36] . These investigations have begun to place the bonding descriptions of An-L bonding on a rigorous, quantitative footing, and have been consistent with the status quo bonding description of early actinides.…”

Probing thePu4+magnetic moment inPuF4withF19

“…Two fits were performed, one in which δ xx , δ yy , and δ zz were allowed to vary freely, and another in which the shift tensor was forced to be axially symmetric as and slightly narrower in width has been observed for UF 4 (∼ 3 mT, corresponding to 4920 ppm). 29 Absolute shielding parameters can be ascertained from the shifts in Table II using data from Hindermann and Cornell, who reported a value of σ 0 = +188.7 ppm for liquid CFCl 3…”

“…The NMR shifts of nuclei interacting with paramagnetic d-or f-electron centers have been discussed in detail by Shulman and Jaccarino. 20 Gabuda et al 29 and Martel et al 35 have proposed that the field dependent anisotropic hyperfine shifts in paramagnetic tetravalent actinide systems can be approximated as a dipolar field produced by localized, unpaired electrons at the An 4+ sites…”

“…proposed by Gabuda et al for the isostructural compound UF 4 29. The non-axial function was statistically superior to the axial function in modeling the spectra, even accounting for the extra adjustable parameter in the former.…”