Overhauser effect Paramagnetism

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. 

We have chosen to dedicate a full chapter to the nuclear Overhauser effect [1,2] (NOE) because its comprehension is of fundamental importance in dealing with nuclear relaxation [3,4], Especially in paramagnetic compounds, it still represents a most powerful technique to detect dipolar connectivities finally it allows the comprehension of two-dimensional spectroscopies based on dipolar coupling. The NOE is the fractional variation of the intensity of a signal when another signal is selectively saturated. 

A modification of the design of the flow cell can also lead to a further increase in sensitivity [7,8]. The addition of a proton RF coil for selective excitation of the protons before entering the 13C flow cell enables the full build-up of the nuclear Overhauser effect in a continuous-flow system (Figure 8.1.5). The combination of this NMR flow cell design, together with the use of immobilized paramagnetic species, could lead to the registration of continuous-flow 13C NMR spectra of separated species. 

The serial publications listed in Section 1.3 contain a number of articles on various aspects of relaxation. The Encyclopedia of NMR includes 13 articles under titles beginning with the word Relaxation and several other related articles, including Nuclear Overhauser Effect," Carbon-13 Relaxation Measurements,100 Paramagnetic Relaxation in Solution,101 Brownian Motion,102 and Spin—Rotation Relaxation Theory.103 Many of these articles are based on density matrix formulations, which can readily be understood with the background provided in Chapter 11. 

Representative and spectra are presented in Figures 5, 6, 7, and 8 for the Amax and Monterey samples. The quantitative reference (peak at 0.0 ppm) in each spectrum is hexamethyldisiloxane ((CH3)3-Si-0-Si(0113)3). A paramagnetic relaxation reagent (the paramagnetic relaxation reagents used were either tris(acetylacetonato) iron (III), Fe(acac)3, or tris(acetylacetonato) chromium (III), Cr(acac)3 at concentrations of 2-6 X lO M). was added to decrease spin lattice relaxation times (Ti s) and suppress nuclear Overhauser effects (5). In addition,... 

Finally, ONP may be achieved with the aid of applied electromagnetic (i.e., radiofrequency, rf or microwave, mw) fields (c), and thus can most simply be considered as extensions of conventional DNP methods these approaches are referred to as radio-frequency induced ONP (RFONP) and microwave-induced ONP (MIONP), respectively. In conventional DNP, magnetization from thermally polarized electron spins (residing, for example, in permanent paramagnetic centers) is driven via hyperfine interactions to surrounding nuclei using resonant or near-resonant AC fields. More specifically, this process is governed by the conventional Overhauser effect in the case of dynamic hyperfine interactions (e.g., in metals or semiconductors),... 

Figure 6.30. Enhancement of the proton NMR signal amplitude (P/P -l) is shown as a ftinction of microwave frequency as taken by the dynamic nuclear polarization experiments, where P and / are the signal amplitude with and without microwave pumping, respectively [143,146,173,174], The Overhauser effect (OE) typical to the mobile spins makes a shaip peak at the Lannor frequency cve of the electron spin but the Solid State Effect (SSE) shows two extrema separated by o from (O charactcrictic with the fixed paramagnetic spins [8], The solid curves are a guide for the eye. (a). t a is-(CH), (A//pp 0.8 G) and , trs-rich (CH)> (Affpp 7.8 G) 1174]. (b) The temperature dependence of the peak-ratio in //ww-(CH), 173], y/x = 0 means pure Overhauser enhancement and y/x— 1, the solid slate effect [173]. Note the change from the mobile above 150 K to the trapped below 150 K, Most of the solitons are trapped below 10 K, (after (a) [174] and (b) [173])...

High-resolution H n.m.r. spectroscopy has been used to probe the conformations of a number of o-ribofuranosylamine derivatives and such rigid molecules as 2,2 -cyclonucleosides and nucleoside 3, 5 -phosphates in aqueous solution. H N.m.r. spectroscopy has also been used to study details of the intramolecular association and conformations of a- and j8-linked pyridine ribo-nucleosides and their 5 -phosphates. The results were analysed in terms of base-D-ribose, o-ribose-side-chain, and base-side-chain interactions and the conformational restraints imposed by the cis HO-2-HO-3 interaction in jS-nucleo-tides and the additional cis HO-2 -base interaction in a-nucleotides. H N.m.r. measurements - including measurements of nuclear Overhauser effects and paramagnetic relaxations effected by Mn + cations - have been used to investigate the preferred conformation about the jV-glycosidic bond of 8-amino-, 8-methyl-amino-, and 8-dimethylamino-adenylic acid, all of which competitively inhibit the coenzyme NADH in the reaction with chicken-muscle lactate dehydrogenase. The primary and secondary amines were shown to prefer anti conformations, whereas the tertiary amine prefers a syn conformation. 

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