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Relativistic heavy-atom effects on heavy-atom nuclear shieldings
Perttu Lantto1, Rodolfo H Romero, Sergio S Gómez
1NMR Research Group, Department of Physical Sciences, University of Oulu, P.O. Box 3000, FIN-90014 Oulu, Finland. perttu.lantto@oulu.fi
The Journal of Chemical Physics
|November 23, 2006
Summary
This study systematically calculates relativistic heavy-atom effects on NMR shielding using ab initio methods. Including third-order spin-orbit contributions is crucial for accurate heavy-atom chemical shift predictions.
Area of Science:
- Computational Chemistry
- Quantum Chemistry
- Nuclear Magnetic Resonance Spectroscopy
Background:
- Relativistic effects significantly influence the electronic structure and properties of heavy atoms.
- Understanding heavy-atom NMR chemical shifts requires accurate theoretical treatment of these relativistic phenomena.
- Previous studies have not systematically investigated the primary relativistic heavy-atom effects (HAHA) on NMR shielding.
Purpose of the Study:
- To systematically calculate the principal relativistic heavy-atom effects (HAHA) on the NMR shielding tensor of heavy atoms.
- To investigate the dependence of HAHA effects on the chemical environment for various heavy-atom systems.
- To compare results obtained using the Breit-Pauli Hamiltonian with fully relativistic Dirac-Hartree-Fock calculations.
Main Methods:
- Ab initio calculations employing the Breit-Pauli Hamiltonian to determine relativistic effects.
- Perturbational relativistic approach to study nuclear shielding and chemical shifts.
- Inclusion of second-order magnetic-field-dependent spin-orbit (SO) shielding contributions, alongside third-order SO and Fermi contact/spin-Zeeman cross-terms (FC/SZ-KE).
Main Results:
- The second-order SO mechanism and FC/SZ-KE are isotropic and largely independent of chemical environment and electron correlation.
- Third-order SO terms are essential for accurate NMR chemical shifts and contribute to shielding anisotropy, significantly differing between BiH(3) and BiF(3).
- FC/SZ-KE contributions to shielding are primarily from heavy atom s orbitals and show universal fractions for core and subvalence shells.
Conclusions:
- While nonrelativistic methods capture trends, reliable heavy-atom NMR chemical shifts necessitate the inclusion of third-order SO contributions.
- Relativistic corrections, particularly third-order SO terms, are vital for accurate predictions of NMR chemical shifts in heavy-atom systems.
- The study provides significant insights into the nature and magnitude of HAHA contributions to absolute shielding in heavy atoms.
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