NMR Shielding Calculations across the Periodic Table:  Diamagnetic Uranium Compounds. 2. Ligand and Metal NMR

In this and a previous article (J. Phys. Chem. A 2000, 104, 8244), the range of application for relativistic density functional theory (DFT) is extended to the calculation of nuclear magnetic resonance (NMR) shieldings and chemical shifts in diamagnetic actinide compounds. Two relativistic DFT methods are used, ZORA (“zeroth-order regular approximation”) and the quasirelativistic (QR) method. In the given second paper, NMR shieldings and chemical shifts are calculated and discussed for a wide range of compounds. The molecules studied comprise uranyl complexes, [UO₂L_n]^±q; UF₆; inorganic UF₆ derivatives, UF_6-nCl_n, n = 0−6; and organometallic UF₆ derivatives, UF_6-n(OCH₃)_n, n = 0−5. Uranyl complexes include [UO₂F₄]^2-, [UO₂Cl₄]^2-, [UO₂(OH)₄]^2-, [UO₂(CO₃)₃]^4-, and [UO₂(H₂O)₅]²⁺. For the ligand NMR, moderate (e.g., ¹⁹F NMR chemical shifts in UF_6-nCl_n) to excellent agreement [e.g., ¹⁹F chemical shift tensor in UF₆ or ¹H NMR in UF_6-n(OCH₃)_n] has been found between theory and experiment. The methods have been used to calculate the experimentally unknown ²³⁵U NMR chemical shifts. A large chemical shift range of at least 21 000 ppm has been predicted for the ²³⁵U nucleus. ZORA spin−orbit appears to be the most accurate method for predicting actinide metal chemical shifts. Trends in the ²³⁵U NMR chemical shifts of UF_6-nL_n molecules are analyzed and explained in terms of the calculated electronic structure. It is argued that the energy separation and interaction between occupied and virtual orbitals with f-character are the determining factors.