NMR shielding calculations across the periodic table: Diamagnetic uranium compounds. 1. Methods and issues

In this and a subsequent article, 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 acti nide compounds. In the given first paper, various issues are explored that are related to this goal. dt is shown that both the relativistic DFT-ZORA ( zeroth-order regular approximation, as developed for NMR properties by Wolf f, S. K.; Ziegler, T.; van Lenthe, E.; Baerends, E. J. J. Chem. Phys. 1999, 110, 7689) and the older quasi-relativistic (QR) DFT methods are applicabl e to these compounds. Another popular relativistic method, the use of relat ivistic effective core potentials (ECP) for the calculation of ligand NMR p arameters, is tested as well. It is demonstrated that the ECP approach is b eyond its limits for the very heavy actinide compounds. Comparing the ZORA and Pauli approaches, it is found that Pauli is mon accurate for the H-1 NM R in UF6-n(OCH3)(n) compounds whereas ZORA is more accurate in other cases. This is in contrast to earlier studies that always showed ZORA to be super ior. The neglect of spin-orbit effects, leading to scalar relativistic appr oximations, is possible in some cases. In other cases, however, spin-orbit cannot be neglected. For instance in UF5(OCH3), a large spin-orbit chemical shift of about 7 ppm has been found for the H-1 nuclei but only small effe cts for the other Ligand nuclei. The large influences of the reference geom etry, the reference compound, and the exchange correlation (XC) functional are demonstrated and discussed. The F-19 chemical shift tensor in UF6 is we ll reproduced by the ZORA and QR methods. However, for the F-19 chemical sh ifts in UF6-nClpi compounds, only some experimental trends could be reprodu ced by the calculations. Possible explanations are discussed, for these sho rtcomings, including the choice of model XC functional.