Positive NOE

Since the equilibrium state has been disturbed, the system tries to restore equilibrium. For this it can use as the predominant relaxation pathways the double-quantum process (in fast-tumbling, smaller molecules), leading to a positive nOe, or the zero-quantum process 1% (in slower-tumbling macromolecules), leading to a negative nOe. 

It is possible to distinguish between direct and indirect nOes from their kinetic behavior. The direct nOes grow immediately upon irradiation of the neighboring nucleus, with a first-order rate constant, and their kinetics depend initially only on the intemuclear distance r" indirect nOes are observable only after a certain time lag. We can thus suppress or enhance the indirect nOe s (e.g., at He) by short or long irradiations, respectively, of Ha- a long irradiation time of Ha allows the buildup of indirect negative nOe at He, while a short irradiation time of Ha allows only the direct positive nOe effects of Ha on He to be recorded. 

The positive nOe observed in small molecules in nonviscous solution is mainly due to double-quantum relaxation, whereas the negative nOe observed for macromolecules in viscous solution is due to the predominance of the zero-quantum 1% cross-relaxation pathway. 

If only single-quantum transitions (h, I2, S], and S ) were active as relaxation pathways, saturating S would not affect the intensity of I in other words, there will be no nOe at I due to S. This is fairly easy to understand with reference to Fig. 4.2. After saturation of S, the fMjpula-tion difference between levels 1 and 3 and that between levels 2 and 4 will be the same as at thermal equilibrium. At this point or relaxation processes act as the predominant relaxation pathways to restore somewhat the equilibrium population difference between levels 2 and 3 and between levels 1 and 4 leading to a negative or positive nOe respectively. 

Fig. 9.12 (A) Cross-relaxation rates with respect to the correlation time and the spectrometer frequency cOq. (B) While only positive NOE (resulting in cross-peaks with inverted sign compared to the diagonal peaks, left) are present in a mixture of free ligands, the addition of the receptor leads to the...

The latter, in contrast to nuclear Overhauser enhancement and exchange spectroscopy (NOESY), always feature positive NOEs (negative cross-peaks with respect to diagonal), eliminating known problems of NOEs vanishing or spin diffusion, depending on correlation time, when high field spectrometers are used for measurements of medium-size compounds. 

Transferred NOE Weak positive NOE Strong negative NOE Weak negative NOE 78-80... 

The stereochemistry of the obtained 1,4-conjugated adduct 61a was assigned to be the structure shown in Fig. 8 by 111-NMR, COSY, and NOESY spectra, and a NOE experimental study. The observation of positive NOE enhancement on the proton combinations of II/ , - II0, Hsax-H0, and... 

Perturbation of the equilibrium population difference for one nucleus (increased spin temperature) spreads over time to perturb the population difference (increase or decrease the spin temperature) of other nuclei that are nearby in space. For small molecules, increasing the spin temperature of one nucleus will decrease the spin temperature of nearby nuclei ( negative NOE ). This leads to an enhancement of peak intensities corresponding to the nearby nuclei. For large molecules, increasing the spin temperature of one nucleus will increase the spin temperature of nearby nuclei ( positive NOE ), leading to a reduction in peak intensity. 

Irie et al.28 obtained experimental evidence for the most stable conformation of the open form by using H NMR nuclear Overhauser effect (NOE) measurements. Irradiation of the protons in the NCH3moiety (3.79 ppm) produced positive NOEs of 10% at the H7 aromatic proton (7.49 ppm) and 19% at the H a olefinic proton. Irradiation of this a hydrogen produced a 12% enhancement of the NCH 3 proton, as illustrated in Figure 2.8. These observations indicated that the geometrical structure of the colored open form of spirooxazine is the TTC form. 

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