Extensions of Magnetic Resonance

A bulk property of a sample called nuclear magnetization, M = (M, My, M ), is the sum of the individual magnetic moment vectors per unit volume. The magnetization can be measured, and its evolution in time can be followed. The characteristic times associated with the time evolution of the magnetization also encode spatial information if the field is nonuniform. There are a number of sophisticated means for measuring these times that involve a pulse sequence in the applied radiation. 

Two sample tubes embedded in a larger sample tube. Spectra are taken in the vicinity of the transition seen for a type of proton center in the substance in the small tubes. The field strength varies in the direction shown by the arrow. As the small tubes are rotated with respect to the applied field, the signal changes from a single peak to two peaks and back again thereby encoding spatial information in the data. 

Two-dimensional H NMR spectrvun of guanosine triphosphate. The small circles are contours of the spectrum s spikes, the equivalent of peaks in a ID spectrum. (They appear as spikes when the intensities are expressed on a scale in the direction perpendicular to the plane of the paper.) Greater intensity is reflected in more contours being shown-more closely spaced contour circles. (From Davis, W.M. and Brown, D., Texas Lutheran University, Seguin, TX. With permission.) 

Electron spin resonance spectroscopy is formally similar to NMR if one considers an unpaired electron as taking the role of a spin -1/2 nucleus in an NMR experiment. However, because of the much greater size of the electron magneton (the Bohr magneton) versus the nuclear magneton, much weaker external fields are employed in order to observe transitions with radiofrequency radiation. As a result, coupling interactions between the electron and magnetic nuclei may require a treatment beyond that of first-order perturbation theory. 

Both ESR and NMR spectra offer information about the electron distribution in a molecule and the geometrical structure of a molecular framework. Both can provide spatial information about types of magnetic nuclei in samples, and that makes the techniques useful for sample imaging. 

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