Nuclear Overhauser effect molecular motion

Summary The utility of the NMR parameters longitudinal relaxation time (Tj) and nuclear Overhauser effect (NOE) for a deeper insight into the molecular structure and motion of polymer siloxanes was tested. A few characteristic examples of siloxanes investigated were presented to show problems and results. 

The aim of this work was to find out how to get more information about stereochemistry and molecular motion of polymer methyl- and methyl-phenyl-siloxanes by measuring longitudinal relaxation times, Tj, and nuclear Overhauser effects, NOE, of the individual building blocks. 

The Overhauser effect has been widely employed as an NMR analysis method in many disciplines ranging from medical to chemical sciences, and broadly refers to the motion-mediated transfer of spin polarization from a species with a higher gyromagnetic ratio (y) to one with a lower gyromagnetic ratio. Because molecular motion is critical for efficient transfer, the Overhauser effect is most commonly observed in liquid samples. The Overhauser effect can be divided into two categories the nuclear Overhauser effect (NOE), where both species are nuclear spins, and Overhauser DNP, where the higher y spin is an unpaired electron. As Overhauser DNP is the focus of this review, some of the terminology and equations are specific to the Overhauser DNP effect. 

There is no straightforward and completely rigorous procedure for determining the relative combinations of the various relaxation mechanisms, except where one mechanism clearly dominates (e.g., if the maximum possible nuclear Overhauser effect (NOE) for a resonance is obtained, dipolar relaxation must dominate its relaxation or an increase in relaxation rate in proportion to the square of the applied field must be due to chemical shift anisotropy). Hence, the study of molecular motion in proteins from relaxation data is performed most readily on nuclei directly bonded to H, and so principally relaxed via dipole-dipole interactions (see Section 4(e)(iii)). 

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. 

E. T. Olejniczak, C. M. Dobson, M. Karplus, and R. M. Levy, /. Am. Chem. Soc., 106, 1923 (1984). Motional Averaging of Proton Nuclear Overhauser Effects in Proteins. Predictions from a Molecular Dynamics Simulation of Lysozyme. 

It is well-known that longitudinal (spin-lattice) relaxation is a blend of a mrmber of different independent mechanisms (73Lev). Dipole-dipole interaction (DD) associated to the nuclear Overhauser effect (NOE) (71Nog) is mostly dominant for nuclei other contributions are spin-rotation (SR) caused by fast molecular or segmental motion, chemical shift anisotropy (CSA) and scalar coupling (SC). 

NMR and EPR techniques provide unique information on the microscopic properties of solids, such as symmetry of atomic sites, covalent character of bonds, strength of exchange interactions, and rates of atomic and molecular motion. The recent developments of nuclear double resonance, the Overhauser effect, and ENDOR will allow further elucidation of these properties. Since the catalytic characteristics of solids are presumably related to the detailed electronic and geometric structure of solids, a correlation between the results of magnetic resonance studies and cata lytic properties can occur. The limitation of NMR lies in the fact that only certain nuclei are suitable for study in polycrystalline or amorphous solids while EPR is limited in that only paramagnetic species may be observed. These limitations, however, are counter-balanced by the wealth of information that can be obtained when the techniques are applicable. 

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