Relativistic Calculations of Solid-state NMR parameters

Solid-State Nuclear Magnetic Resonance (NMR) spectroscopy is a powerful experimental probe of structure and dynamics on an atomic scale. It has been widely applied to problems in chemistry, material science, biology, physics, and geology. However, there is no simple theorem which allows the measured NMR spectrum to be related to the underlying chemical structure. For simple organic molecules and certain crystal structures empirical rules have been found, but for more complex systems interpretation of the experimental spectra can be difficult and often ambiguous.
First principles quantum mechanical calculations of NMR parameters have the potential to provide the vital missing link between NMR spectra and the underlying microscopic structure. This challenge has led to the development of the Gauge Including Projector Augmented Wave (GIPAW) method (http://www.gipaw.net) which enables NMR parameters to be calculated within the planewave-pseudopotential formalism of density functional theory (DFT). The translational symmetry found in crystalline materials is specifically included within this method, although it can also be applied to aperiodic materials using a supercell approach.
The ability to predict from first-principles NMR parameters for solid-state systems has had a significant impact on the solid-state NMR community. Such calculations are often an integral part of any experimental solid-state NMR study. However, a major limitation is the poor description of compounds containing heavier elements (roughly speaking those beyond Tellurium). This applies not just to the heavy atom itself, but to any light atoms (H, C) directly bonded to the heavier atom (the so called 'heavy atom - light atom effect) The reason for this is a neglect of relativistic effects which become important for increasing atomic number. But simply the heavier the atom, the deeper the potential the inner electrons experience, and the faster their speed. For moderately heavy atoms the inner electrons travel at an appreciable fraction of the speed of light. While so-called scalar relativistic effects are sufficient in some situations, a full treatment including spin-orbit coupling is essential to predict phenomena such as the heavy atom - light atom effect. We have recently extended the CASTEP code to include spin-orbit coupling in the calculation of ground state properties. The aim of this project will be to apply this functionality to the calculation of NMR properties in solids - enabling the accurate prediction of NMR parameters across the periodic table. This will involve the development of new theoretical equations and their implementation into a parallel electronic structure code (CASTEP http://www.castep.org). Applications of the new methodology will be extensive - and include areas such as catalysis, geominerals and pharmaceuticals, with collaborations in both academia and industry.

EPSRC Classification: Computational and Theoretical Chemistry.

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