Quadrupole Couplings in Nuclear Magnetic Resonance, General
Nuclear Magnetic Resonance and Electron Spin Resonance Spectroscopy
Published Online: 15 SEP 2006
Copyright © 2000 John Wiley & Sons, Ltd. All rights reserved.
Encyclopedia of Analytical Chemistry
How to Cite
Man, P. P. 2006. Quadrupole Couplings in Nuclear Magnetic Resonance, General. Encyclopedia of Analytical Chemistry.
- Published Online: 15 SEP 2006
Nuclear magnetic resonance (NMR) spectroscopy is continually finding new applications. It enables the local symmetry to be probed at the atomic scale using the nuclear spins I of the compound under investigation. The nuclear spin is either a half-integer (or odd) number or an integer (or even) number. The nuclei in the periodic table can be divided into two parts – spin- nuclei and spin larger than nuclei. The spin larger than nuclei are called quadrupole nuclei because they possess an electric quadrupole moment which interacts with the electric-field gradient (EFG) generated by its surroundings. By extension, their spins are called quadrupole spins. Spin- nuclei are not sensitive to the EFG. Of the nuclei that possess a spin, 6% have integer quadrupole spins and 66% have half-integer quadrupole spins.
This article focuses on the half-integer quadrupole spins (, and ) in single crystals and in powder compounds. Most of these spins are observable. As they are multi-energy-level systems (the number of energy levels is 2I + 1), multiple quantum (MQ) transitions occur during excitation of the spin system by a radiofrequency (RF) pulse sequence. As a result, quantum mechanical concepts are needed for an understanding of the spin dynamics and for interpretation of the results. In particular, the choice of pulse sequence and the experimental conditions, such as pulse duration, pulse strength, and phase cycling in the pulse sequence, depend on the strength of the EFG surrounding the nuclear spin.