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Paramagnetic behavior of Actinide ions in NMR spectroscopy

Abstract : Because of uncomplete filling of the 5f orbitals, most of the Actinides ions (An) have a paramagnetic behavior (except Ac3+, Th4+. UO22+ which are 5f0). It means that in the magnetic field B0 of a NMR spectrometer a magnetic moment is induced changing the Magnetic Susceptibility (MS) of the liquid sample containing the An ion. Secondly, the electronic spin resulting from the 5f orbitals interacts with nuclear spins of the ligand bound to the An cation. This well-known phenomenon has been unraveled and described for the Lanthanide cations (Ln) at the oxidation state 3 in solution since the 70s.[1] Consequently, it turns out the NMR spectroscopy is an original way to probe the 5f electron behaviors of An ions provided an spectrometer is available in the lab for the use of radioactive samples[2]. Both phenomenona are easily quantified from chemical shifts δexp measurements induced by a paramagnetic An ion: δexp = δbulk + δdia + δpara From δbulk measurements through the Evans method, MS of An at different oxidation state have been collected all in the same aqueous conditions. The values are found in good agreement with values published in the 50s[3] on solid state samples. This NMR technique proved to be reliable enough so that radioactivity effects on MS measurements are brought out for weakly paramagnetic cations such as Pu3+ and Am3+[4]. Conversely to Ln(III) cations and the free ions model, quantum chemistry methods are required to explain the paramagnetic properties of An cations in aqueous phase. It is however difficult to calculate MS changes arising from chloride or nitrate anions when added in the experimental media. The interactions between the electronic spin of An ion and the nuclear spins of a ligand bound to it are depicted through the δdia+δpara experimental values. In the case of the Ln(III) cations the paramagnetic effects on the chemical shifts are quite well rationalized owing to Bleaney’s equation so that geometric information can be deduced from the induced paramagnetic shifts[1]. The use of An cations as paramagnetic probe like with the Ln(III) series could be a good opportunity to reach geometric information on An complexes in solution. However and as observed with the An MS, the crystal-field effects from ligands are of greater importance compared to Ln(III). Consequently some assumptions made by Bleaney are no more valid. Consequently it is of main concern to check Bleaney’s equation validity applied to An cations and to suggest to what extent the equation can be used. From a theoretical point of view it is helpful to better understand the 5f electron behaviors. It could be an efficient tool to get structural information about An complexes in solutions but also an opportunity to grab information that could make possible a kind of covalency quantification between Ln and An ions. This communication will give an overview of the information deduced from MS measurements and quantum chemistry calculations. Regarding the induced paramagnetic chemical shifts, the use of Bleaney equation applied to An complexes with the dipicolinic acid will be discussed pointing out accountered issues in this field.
Keywords : NMR spectroscopy
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Contributor : Bibliothèque Cadarache <>
Submitted on : Wednesday, October 30, 2019 - 9:20:16 AM
Last modification on : Tuesday, April 28, 2020 - 11:28:14 AM

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  • HAL Id : cea-02338554, version 1

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C. Berthon, Mc. Illy, P. Moisy, L. Guerin, H. Bolvin. Paramagnetic behavior of Actinide ions in NMR spectroscopy. ATAS2018, Nov 2018, Nice, France. ⟨cea-02338554⟩

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