Nuclear relaxation processes

The NMR phenomenon can be quantitatively described in classical terms. This was first done by Bloch.The approach is helpful in developing an understanding of nuclear relaxation processes. 

Unpaired electronic density can be delocalized onto the various nuclei of the complex via through-bond scalar hyperfine interactions involving occupied orbitals containing s-character (direct interaction or polarization according to the Fermi mechanism, Wertz and Bolton (1986)). Random electron relaxation thus produces a flip-flop mechanism which affects the nuclear spin and increases nuclear relaxation processes (Bertini and Luchinat, 1996). Since these interactions are isotropic, they do not depend on molecular tumbling and re is the only relevant correlation time for non-exchanging semi-rigid complexes. Moreover, only electronic spin can be delocalized via hyperfine interactions (no orbital contribution) and the contact re-... 

Under continuous uv irradiation, the observed steady-state polarization (whether by cw or by FT spectrometers) may be substantially modified by various nuclear relaxation processes. For example, Closs and Czeropski (35,36) have demonstrated that CIDNP can be transferred from a group of polarized nuclei to another group not originally polarized. Both the dipolar and the scalar relaxation mechanisms (of the nuclear Overhauser effects) can be operative. The extremely interesting case of intramolecular dipolar nuclear cross relaxation reported by Closs and Czeropski (35) involves the thermal reaction of... 

In many ways the information available from NMR studies on these two isotopes is complementary. Although 14N is by far the more common, in natural abundance, it has an electric quadrupole moment while the less abundant 15N nucleus does not. Thus the l4N nucleus has been more frequently used in chemical shift studies on natural abundance samples as well as in quadrupolar relaxation investigations. The larger magnetogyric ratio of the l5N nucleus, together with its sharper lines, renders it more suitable for studies involving spin-spin coupling constants in addition to those on the less efficient nuclear relaxation processes. 

FIGURE 19-7 Two nuclear relaxation processes. Longitudinal relaxation lakes place in the IV plane transverse relaxation in Ihe < v plane. (Courlesy of Professor Stanford L Smith. University of Kentucky, Lexington, KY, ... 

Chemical information obtained using and C NMR is usually obtained on samples in solution (liquid-state NMR) in order to improve resolution. However, C spectra can also be obtained on neat specimens, such as rubber. This is possible as long as there is sufficient molecular motion to average the orientation-dependent variation in chemical shift of chemically identical atoms (chemical shift anisotropy, CSA). Chemical shifts in C NMR spectra span a much wider range than in proton NMR, and therefore the former provides better spectral resolution. However, the Nuclear Overhauser effect (NOE) and other nuclear relaxation processes cause the C absorption intensities to deviate from direct proportionality to the number of carbon atoms. Thus, unless specific techniques are utilized, C NMR spectral intensities using standard liquid-state NMR acquisition methods are not quantitative. 

The electron has a magnetogyric ratio that is more than 600 times larger than that of the proton thus, if unpaired electrons are present their dipole-dipole interaction with a given nucleus normally controls the relaxation of that nucleus. Consequently, paramagnetic centres may be introduced to override nuclear-nuclear relaxation processes in certain cases, for example to reduce embarrassingly long relaxation times and to remove NOEs in cases where they are not required. 

By extrapolation of the role of lattice phonons in nuclear relaxation processes in solids it is not a great step to appreciate that the application of ultrasound to both solids and liquids may be used to manipulate phenomena of interest to NMR spectroscopists. Although not yet an area of considerable activity the relatively few examples of the combined use of NMR and ultrasound that will now be described indicate that further such studies will prove profitable. 

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