Density functional calculations of F-19 and U-235 NMR chemical shifts in uranium (VI) chloride fluorides UF6-nCln: Influence of the relativistic approximation and role of the exchange-correlation functional

摘要

Relativistic density functional theory (DFT) has been applied to the calculation of the F-19 nuclear magnetic resonance (NMR) chemical shifts of the title compounds. It is shown that, while large-core effective core potentials (ECP) fail completely for the calculation of ligand NMR chemical shifts in uranium compounds, small-core ECPs are a valid relativistic method for this purpose. In an earlier study of the same systems, certain differences between theory and experiment had been observed, for instance, in the relative chemical shift of the A(4) and X sites in UF5Cl. The reason for these deviations has been investigated further in the current paper. By comparing different relativistic methods, it is shown that the relativistic approximation is not responsible for these deviations. The role of the approximation to the exchange-correlation (XC) functional of DFT has been probed, and generalized gradient approximations (GGA) as well as hybrid DFT methods have been investigated. None of these methods corrects the mentioned errors. It is argued that the neglect of environmental factors (solvent effects) remains as a possible error source, although the approximate XC functional appears to be the more likely cause of the problem. U-235 NMR shieldings and chemical shifts have been calculated, and the trends predicted earlier have been confirmed. (C) 2004 Wiley Periodicals, Inc.