Calculation of electric field gradients in Cd (II) complexes

Calculated electric field gradients (EFGs) are instrumental in the interpretation of experimentally determined nuclear quadrupole interactions (NQIs), in terms of electronic and molecular structure. Obtaining accurate calculated EFGs is therefore central for studying the structure of many types of molecules. In a recent work, researchers from the University of Copenhagen studied model systems for the CueR protein.

The functional BHandHLYP was chosen after testing various DFT functionals against CCSD(T) for calculation of EFGs at the position of Cd(II) in Cd(SCH₃)₂. The influence of relativistic effects using both the scalar relativistic and spin-orbit relativistic Zeroth-Order Regular Approximation (ZORA) Hamiltonian, the basis set size, and the size of the CueR protein model systems, were assessed using ADF. Truncating the model systems at the first C–C bond from the central Cd(II) was inadequate to obtain reliable EFG properties; a larger model system must therefore be employed to achieve reliable EFG calculations. The maximum difference between non-relativistic and scalar relativistic ZORA calculations amounted to 0.28 a.u. or 9% of Vzz. Inclusion of spin orbit coupling led to a further increase of Vzz, rising in magnitude by 0.05 a.u. From the results, it was evident that the spin–orbit contribution was insignificant, but the scalar component was crucial. Spin-orbit ZORA calculations with the BHandHLYP functional and a locally dense basis set (a combination of the basis sets QZ4P, TZ2P and DZP) provide reliable results, and allowed for structural interpretation of the experimental data.

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Catriona A. O’Shea, Rasmus Fromsejer, Stephan P. A. Sauer, Kurt V. Mikkelsen and Lars Hemmingsen, Calculation of electric field gradients in Cd(ii) model complexes of the CueR protein metal site, Phys. Chem. Chem. Phys., 2023, 25, 12277-12283