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.