Spin-locking of half-integer quadrupolar nuclei in NMR of solids: the far off-resonance case

Odedra, S. and Wimperis, S. (2017) Spin-locking of half-integer quadrupolar nuclei in NMR of solids: the far off-resonance case. Solid State Nuclear Magnetic Resonance, 84, pp. 4-13. (doi: 10.1016/j.ssnmr.2016.11.001)

Abstract

Spin-locking of spin I=3/2 and I=5/2 nuclei in the presence of large resonance offsets has been studied using both approximate and exact theoretical approaches and, in the case of I=3/2, experimentally. We show the variety of coherences and population states produced in a far off-resonance spin-locking NMR experiment (one consisting solely of a spin-locking pulse) and how these vary with the radiofrequency field strength and offset frequency. Under magic angle spinning (MAS) conditions and in the “adiabatic limit”, these spin-locked states acquire a time dependence. We discuss the rotor-driven interconversion of the spin-locked states, using an exact density matrix approach to confirm the results of the approximate model. Using conventional and multiple-quantum filtered spin-locking 23Na (I=3/2) NMR experiments under both static and MAS conditions, we confirm the results of the theoretical calculations, demonstrating the applicability of the approximate theoretical model to the far off-resonance case. This simplified model includes only the effects of the initial rapid dephasing of coherences that occurs at the start of the spin-locking period and its success in reproducing both experimental and exact simulation data indicates that it is this dephasing that is the dominant phenomenon in NMR spin-locking of quadrupolar nuclei, as we have previously found for the on-resonance and near-resonance cases. Potentially, far off-resonance spin-locking of quadrupolar nuclei could be of interest in experiments such as cross polarisation as a consequence of the spin-locking pulse being applied to a better defined initial state (the thermal equilibrium bulk magnetisation aligned along the z-axis) than can be created in a powdered solid with a selective radiofrequency pulse, where the effect of the pulse depends on the orientation of the individual crystallites.

• Item Type: Articles
• Status: Published
• Refereed: Yes
• Glasgow Author(s) Enlighten ID: Odedra, Dr Smita and Wimperis, Professor Stephen
• Authors: Odedra, S., and Wimperis, S.
• College/School: College of Science and Engineering > School of Chemistry
• Journal Name: Solid State Nuclear Magnetic Resonance
• Publisher: Elsevier
• ISSN: 0926-2040
• ISSN (Online): 1527-3326
• Published Online: 30 November 2016