Novel half-integer quadrupolar solid-state NMR correlation experiments for probing atomic proximities and connectivities in disordered materials

Lead Research Organisation: University of Warwick
Department Name: Physics

Abstract

When scientists investigate problems, like all good detectives they need clues as to what is happening. For a whole range of key problems, techniques that can reveal the local environment around an atom are crucial to provide insight into the structure at this level, which often governs how a material or molecule behaves. Nuclear Magnetic Resonance (NMR) spectroscopy has increased in importance throughout the sciences as it is an element-specific probe that can distinguish very small changes in the surroundings of different sites (e.g. whether a boron atom is bonded to three or four oxygen atoms and hence adopts a trigonal or a tetrahedral arrangement). NMR exploits the inherent magnetism of atomic nuclei which are at the centre of all atoms: like the alignment of a compass needle in the Earth's magnetic field, nuclear magnets have a preferred direction when placed in a strong magnetic field. This preference, however, is weak and a nuclear magnet can be made to change its direction from, e.g., being aligned with to being aligned against the direction of the magnetic field, by applying a resonant radio wave, i.e., one whose frequency and hence energy matches precisely the energy required to flip the nuclear magnet. The electrons surrounding the atomic nucleus are also inherently magnetic and are affected by the presence of a magnetic field. Importantly, the resonant frequency of a particular nucleus depends very sensitively on this additional response of the electrons, such that the atomic nuclei act as spies of the local electron environment and hence the specific chemical bonding, allowing it to be used to probe environments as described above. The resonant frequency of different nuclear isotopes are well separated such that an NMR spectrum is specific to a particular chosen isotope. (An element can exist as different isotopes whereby there is the same number of protons but a different number of neutrons in the nucleus.) This project considers so-called quadrupolar nuclei which have a quadrupole electronic moment (i.e., there is a non-uniform distribution of electric charge in the nucleus). Over two-thirds of all isotopes are such quadrupolar nuclei, and many important elements, e.g., lithium, boron, oxygen, sodium, aluminium only have NMR-active isotopes that are quadrupolar. Quadrupolar nuclei are often difficult because the strong interaction of the quadrupole moment with the environment generated by the electrons leads to broad lines in NMR spectra. One of the key advantages for NMR is that nuclei experience interactions that convey information about their surroundings. As an example, the dipole interaction arises as the nuclear magnets are not isolated, but rather they interact in an analogous way to how two bar magnets either attract or repel when brought close together. A related interaction is the J coupling where the electrons between the nuclei enable one nucleus to sense another nucleus to which it is chemically bonded. This project will develop new NMR experiments applicable to solid samples that use dipolar interactions and related J couplings to identify through-space proximities or through-bond connectivities between quadrupolar nuclei. A test of a good technique is that it is applicable to a wide range of problems. In this project, the new NMR experiments will be used to determine the atomic-scale structure of glasses that have applications in batteries, dental cement, ovenware, telescope mirrors, and radioactive waste immobilisation. There is always a link between the bulk structure of a material and its hidden atomic-scale structure, hence a better understanding of the latter will enable better materials to be developed. It is through the partnership between problem-based and technique-based scientists that real progress is made.