Rovibrational effects on NMR shieldings in a heavy-element system: XeF2

Abstract

Fully quantum-mechanical treatment of the effects of thermal rovibrational motion in a heavy-element molecule with relativistic effects is carried out for the heavy (129/131)Xe and light (19)F nuclear shieldings in the linear XeF(2) molecule. More importantly, purely quantum-mechanical, intramolecular phenomena, the primary and secondary isotope effect on these shieldings, respectively, are treated with including both the zero-point vibrational and finite-temperature effects. While large solvent effects influence the experimental absolute shielding constants and chemical shifts (thereby making comparison of experiment and theory very difficult), they are not significant for the isotope shifts. We study the role of electron correlation at both nonrelativistic (NR) and relativistic [Breit-Pauli perturbational theory (BPPT) as well as 4-component Dirac theory] level. We obtain quantitative agreement with the nearly solvent-independent experimental (19)F secondary isotope shifts. This implies a promising accuracy for our predictions of the experimentally so far non-existing primary Xe isotope shift and the temperature dependence of Xe and F chemical shifts corresponding to a low pressure gas phase. To achieve this, a combination of high-level ab initio NR shielding surface is found necessary, in the present work supplemented by relativistic corrections by density-functional theory (DFT). Large errors are demonstrated to arise due to DFT in the NR shielding surface, explaining findings in recent computational studies of heavy-element isotope shifts. Besides a high-quality property hypersurface, the inclusion of thermal effects (in addition to zero-point motion) is also necessary to compare with experimental results. The geometry dependence of the different relativistic influences on the wave function, Zeeman interaction, and hyperfine interaction, as well as their role in the temperature dependence of both the Xe and F shielding constants and their isotope shifts, are discussed. The relativistic rovibrational effects arise from the same individual contributions as previously found for the chemical shifts and shielding anisotropies. In general, the spin-orbit interactions are more sensitive to rovibrational motion than the scalar relativistic contributions. A previously suggested third-order BPPT contribution to shielding anisotropy is shown to be important for a better agreement with experiment.

Electron correlation and relativistic effects in the secondary NMR isotope shifts of CSe2

“…In a solvated molecule the latter properties are much more sensitive to intermolecular interactions, which are challenging to model quantitatively. [2][3][4][5] Accounting for zero-point vibrational (ZPV) corrections is increasingly routine in the modeling of NMR spectroscopic parameters. [6][7][8] Although there exist semi-automated methods for treating ZPV in connection with nonrelativistic (NR) computations, only a few studies have been published on nuclear motion effects on relativistic (REL) heavy-element NMR shieldings.…”

“…[6][7][8] Although there exist semi-automated methods for treating ZPV in connection with nonrelativistic (NR) computations, only a few studies have been published on nuclear motion effects on relativistic (REL) heavy-element NMR shieldings. 5,[9][10][11][12][13] Furthermore, a treatment of finite-temperature effects is far less common: especially comprehensive quantum-mechanical treatments of heavy-element compounds are few and far between. 5 So far, the finite-temperature effects on magnetic properties, such as magnetizabilities and rotational g-tensors 14 as well as on the NMR shieldings and their isotope shifts, [3][4][5][15][16][17] have usually been demonstrated for light elements.…”

“…5,[9][10][11][12][13] Furthermore, a treatment of finite-temperature effects is far less common: especially comprehensive quantum-mechanical treatments of heavy-element compounds are few and far between. 5 So far, the finite-temperature effects on magnetic properties, such as magnetizabilities and rotational g-tensors 14 as well as on the NMR shieldings and their isotope shifts, [3][4][5][15][16][17] have usually been demonstrated for light elements. The main relativistic corrections to light-atom NMR shieldings due to spin-orbit (SO) interactions [18][19][20][21] have long been known to be sensitive to rovibrational motion.…”

“…28 Concerning heavy-atom systems there is mounting evidence obtained by benchmarking DFT results against coupled-cluster singles and doubles with perturbative triples [CCSD(T)] data that the NR shielding is, in fact, more sensitive to the electron correlation treatment than the REL contribution. 5,[29][30][31][32] DFT shielding calculations of heavy nuclei appear to be susceptible to (at times) appreciable errors originating already at the NR level. In particularly difficult electron correlation cases also the relativistic corrections themselves should be computed at a correlated ab initio level.…”

Nuclear spin-spin coupling in a van der Waals-bonded system: Xenon dimer

“…In addition to the approximate ZORA methodology mentioned above, 9, 10, 15 DFTbased, fully relativistic four-component methods are now available for J-coupling in the DIRAC software. 16 However, the presently used DFT functionals have not been found very reliable for the NMR properties of xenon [21][22][23][24] or intermolecular couplings, 13 mainly due to deficiencies appearing already at the nonrelativistic (NR) level as calibrated by highly correlated ab initio calculations. Therefore, we investigate also hybrid levels of theory in which such correlated NR calculations (currently unavailable at the relativistic level) are combined with relativistic effects obtained as differences between fully relativistic and NR calculations at the DFT and uncorrelated Hartree-Fock levels.…”