Nuclear Magnetic Resonance Measurements and Electronic Structure of Pu(IV) in [(Me)₄N]₂PuCl₆

The synthesis, electronic structure, and characterization via single-crystal X-ray diffraction, nuclear magnetic resonance (NMR) spectroscopy, and magnetic susceptibility of (Me₄N)₂PuCl₆ are reported. NMR measurements were performed to both search for the direct ²³⁹Pu resonance and to obtain local magnetic and electronic information at the Cl site through ³⁵Cl and ³⁷Cl spectra. No signature of ²³⁹Pu NMR was observed. The temperature dependence of the Cl spectra was simulated by diagonalizing the Zeeman and quadrupolar Hamiltonians for ³⁵Cl, ³⁷Cl, and ¹⁴N isotopes. Electronic structure calculations predict a magnetic γ₅ triplet ground state of Pu(IV) in the crystalline electric field of the undistorted PuCl₆ octahedron. A tetragonal distortion would result in a very small splitting (∼20 cm^-1) of the triplet ground state into a nonmagnetic singlet and a doublet state. The Cl shifts have an inflection point at T ≈ 15 K, differing from the bulk susceptibility, indicating a nonmagnetic crystal field ground state. The Cl spin-lattice relaxation time is constant to T = 15 K, below which it rapidly increases, also supporting the nonmagnetic crystal field ground state.