Nuclear magnetic resonance shielding constants and chemical shifts in linear 199Hg compounds: A comparison of three relativistic computational methods

Abstract

We investigate the importance of relativistic effects on NMR shielding constants and chemical shifts of linear HgL(2) (L = Cl, Br, I, CH(3)) compounds using three different relativistic methods: the fully relativistic four-component approach and the two-component approximations, linear response elimination of small component (LR-ESC) and zeroth-order regular approximation (ZORA). LR-ESC reproduces successfully the four-component results for the C shielding constant in Hg(CH(3))(2) within 6 ppm, but fails to reproduce the Hg shielding constants and chemical shifts. The latter is mainly due to an underestimation of the change in spin-orbit contribution. Even though ZORA underestimates the absolute Hg NMR shielding constants by ∼2100 ppm, the differences between Hg chemical shift values obtained using ZORA and the four-component approach without spin-density contribution to the exchange-correlation (XC) kernel are less than 60 ppm for all compounds using three different functionals, BP86, B3LYP, and PBE0. However, larger deviations (up to 366 ppm) occur for Hg chemical shifts in HgBr(2) and HgI(2) when ZORA results are compared with four-component calculations with non-collinear spin-density contribution to the XC kernel. For the ZORA calculations it is necessary to use large basis sets (QZ4P) and the TZ2P basis set may give errors of ∼500 ppm for the Hg chemical shifts, despite deceivingly good agreement with experimental data. A Gaussian nucleus model for the Coulomb potential reduces the Hg shielding constants by ∼100-500 ppm and the Hg chemical shifts by 1-143 ppm compared to the point nucleus model depending on the atomic number Z of the coordinating atom and the level of theory. The effect on the shielding constants of the lighter nuclei (C, Cl, Br, I) is, however, negligible.

Computational 19F NMR. 1. General features

“…Whereas it is by no means the only one available, ZORA has the advantage of being the most widely employed one (thanks also to the availability of implementations that include both scalar effects and spin-orbit coupling on NMR parameters), thereby allowing for a comparison with existing data. It should be remarked, however, that other methods have been implemented (see e.g., [110] for a comparison of such methods for 199 Hg). Probably the only instance where relativistic effects should be seriously called into play concerns uranium chlorofluorides; indeed, the rather poor performance of all methods considered here (or in the literature at large) might be due in part to the need of improving their treatment, which may be challenging for actinides.…”

Relativistic and Electron-Correlation Effects on the Nuclear Magnetic Resonance Shieldings of Molecules Containing Tin and Lead Atoms

“…The conclusion that DFT gives lower shielding constants compared to HF has been reported previously. 39,[50][51][52][53][54] In addition, hybrid functionals containing exact HF exchange lead to smaller reductions compared to GGA/BP86 and therefore give results which are in better agreement with HF, especially PBE0 which has a larger percentage of HF exchange than B3LYP.…”

Relativistic DFT Calculations of Hyperfine Coupling Constants in 5d Hexafluorido Complexes: [ReF6]2− and [IrF6]2−

“…For example, previous studies showed that, in ZORA calculations of nuclear magnetic shielding constants, it was important to use the large QZ4P basis sets 31, 32. Therefore, we also studied the basis set dependence of both the ZORA and DKS results for the hyperfine coupling constants in both ions.…”