New information on the relativistic effects on heavy-atom nuclear shieldings

31.08.2007

Perttu Lantto and the NMR Research Group at the University of Oulu have established how heavy nucleus induced relativistic phenomena in molecular electronic structure affect the chemical shift of the heavy atom itself. The parallel computation capacities provided by the CSC, the Finnish IT center for science, were efficiently used with the well-parallelizable DFT code (DALTON).

Nuclear magnetic resonance (NMR) spectroscopy is a basic method used in molecular structural analysis. The NMR spectrum of the measured substance provides information on the chemical environment of the detected nucleus, which is extracted from the chemical shift of the resonant frequency of the nucleus. In addition, the coupling parameters determined from the spectrum reveal information on, for example, the spatial structure of the molecule. Magnetic resonance imaging (MRI) is a major NMR application.

Perttu Lantto and the NMR Research Group at the University of Oulu applied the recently developed Breit-Pauli perturbation theory (BPPT) for NMR nuclear shielding parameter in explaining the relativistic phenomena due to heavy nucleus in molecular electron structure affecting the experimentally observed chemical shift of the heavy nucleus itself. As an example, the group used the xenon nucleus, of which chemical shift is especially sensitive to the environment. Together with exceptionally wide shift range, it makes Xe-NMR as an important experimental method for research of materials and biosystems.

In addition to the non-relativistic nuclear shielding mechanisms, the BPPT theory gives a total of 16 relativistic interactions, all of which can be interpreted based on known non-relativistic concepts. The group has shown that in all of the systems studied, the experimentally observed chemical shift is almost solely described by a combination of the non-relativistic and five relativistic nuclear shielding interactions. These five interactions, as well as the non-relativistic shielding, are also extremely sensitive to electron correlation effects, unlike the other 11 factors that can be even calculated by uncorrelated Hartree-Fock theory.

When the results obtained by electron correlation including density functional theory (DFT) methods were compared to the results from the state-of-the-art correlated ab initio wave function calculations, it could be concluded that in calculating the Xe chemical shift, especially the non-relativistic chemical shift created problems for the DFT. In contrast, due to the mutual error cancellation in the BPPT terms, the DFT can be used for evaluation of the total relativistic effect in neutral molecules. The DFT calculations were carried out by the well-parallelized molecular electronic structure code (DALTON), which efficiently utilized the parallel computation capacities provided by the CSC, the Finnish IT center for science.

Photo:
NMR chemical shift of ¹²⁹Xe relative to free Xe atom.  Spectra calculated using various methods of quantum chemistry are compared with experimental results (EXP). AI represents a combination of the best non-relativistic (NR) and relativistic (BPPT) calculations.

More information

• Perttu Lantto, NMR Research Group, University of Oulu, tel. +358 (0)8 553 1312, email: perttu.lantto at oulu.fi
• P. Manninen, K. Ruud, P. Lantto, and J. Vaara, Leading-order relativistic effects on nuclear magnetic resonance shielding tensors, Journal of Chemical Physics 122, 114107:1-8 (2005). Erratum Ibid. 124, 149901:1-2 (2006).
• P. Lantto and J. Vaara, 129Xe chemical shift by the perturbational relativistic method: Xenon fluorides, Journal of Chemical Physics, 127, 084312:1-9 (2007).
  
  

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