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Nuclear Overhauser effect, removal

Finally, Spiess and co-workers49 have shown how the nuclear Overhauser effect (NOE) can be used to study motions in soft solids, such as amorphous polymers. H- H dipolar coupling is already partially removed in such samples... [Pg.30]

NMR has found more recent use in comparative studies of lysozyme and a-lactalbumin. Poulsen et al. (1980), using the nuclear Overhauser effect, were the first to demonstrate the existence of the hydrophobic box region, in solution, for lysozyme, first noted in the crystalline state by Blake et al. (1967a). Koga and Berliner (1985) applied the nuclear Overhauser effect to the study of a-lactalbumin. They too found a hydrophobic box, similar to that of lysozyme. The study was conducted in both the presence and absence of Ca(II), and there were only subtle differences upon removal of the cation. [Pg.266]

Based on the empirical results/ X-ray-diffraction data/ and solution-phase NMR experiments/ a transition state model (6) has been advanced to explain the observed enantioselectivity. The presence of an ortho substituent in the A arylmaleimide reactant directs aluminum coordination to occur with the lone pair of electrons anti to the nitrogen atom. A 3,5-dimethylphenyl moiety present on the ethylenediamine framework blocks one face of the dienophile, resulting in approach of the diene from the backside. A considerable amount of spectroscopic evidence, most notably that obtained fromNOE (nuclear Overhauser effect) experiments, has been accumulated to support this model. A -arylmaleimide derivatives that lack an ortho substituent and other dienophiles (e.g., maleic anhydride) can coordinate to the aluminum catalyst in alternative modes such that the reactive olefin is far removed from the chiral environment of the ligand scaffold, thereby resulting in cycloaddition reactions that exhibit little or no enantioselectivity. [Pg.52]

The 2D NOESY (nuclear overhauser effect spectroscopy) experiment is widely used to measure the NOE between all possible proton pairs. The NOESY experiment consists of three 90° pulses. The first two are used to label the frequencies and to remove each of the proton sites from equilibrium. The NOE develops during the mixing time separating the second and third pulses. The last pulse generates observable signal. [Pg.273]

A pulse interval of 10 s is sufficient to satisfy the spin-lattice relaxation time, T of the methyl group, which is the slowest relaxing nucleus with a Tj of 2 s at 125°C. The number of transients accumulated depended upon the ethylene content and in each case, accumulations are allowed to continue until a satisfactory signal-to-noise ratio was achieved. Each 13C NMR spectrum is recorded with proton noise-decoupling to remove unwanted 13C - IH scalar couplings. No corrections are made for differential nuclear Overhauser effects since these have been shown to be constant for the major resonances in low ethylene - propylene copolymers. ... [Pg.410]

If in organosilicon compounds, the signal splittings caused by the coupling with the protons are removed by decoupling, then a modulation of the signal intensity occurs that is connected with the dipolar term. If the silicon relaxes by dipolar interaction with the protons only, then the signal intensity is multiplied by a factor of -1.52 from the nuclear Overhauser effect (NOE) ... [Pg.875]

Such pulse programs are also used to enable other special one-dimensional experiments such as saturation or nonexcitation of a large solvent resonance (these are different in that the former method will also saturate NH or OH protons in the molecules under study through the mechanism of chemical exchange), or the measurement of nuclear Overhauser enhancement (NOE) effects which are often used to provide distinction between isomeric structures or to provide estimates of internuclear distances. Pulse programs are also used for measuring NMR spectra of nuclei other than and sometimes in order to probe connectivity between protons and the heteronucleus. In this case, pulses or irradiation can be applied on both the heteronucleus and channels in the same experiment. The commonest use is in NMR where all spin-spin couplings between the nuclei and nuclei are removed by... [Pg.420]


See other pages where Nuclear Overhauser effect, removal is mentioned: [Pg.103]    [Pg.111]    [Pg.310]    [Pg.100]    [Pg.323]    [Pg.253]    [Pg.323]    [Pg.138]    [Pg.91]    [Pg.20]    [Pg.251]    [Pg.47]    [Pg.500]    [Pg.124]    [Pg.146]    [Pg.117]    [Pg.120]    [Pg.132]    [Pg.389]    [Pg.216]    [Pg.283]    [Pg.410]    [Pg.10]    [Pg.18]    [Pg.140]    [Pg.39]    [Pg.482]    [Pg.852]    [Pg.485]    [Pg.230]    [Pg.862]   


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