Negative NOE

In decoupling the methyl protons, the NOE difference spectrum shows a nuclear Overhauser enhancement on the cyclopropane proton at = 1.60 and on the terminal vinyl proton with trans coupling at <5// = 5.05 and, because of the geminal coupling, a negative NOE on the other terminal proton at Sh= 4.87. This confirms the trans configuration G. In the cis isomer H no NOE would be expected for the cyclopropane proton, but one would be expected for the alkenyl-// in the a-position indicated by arrows in H. 

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. 

If Ha, Hb, and do not lie on the same line (Fig. 4.6), then as Ha comes closer to He, the direct -fve nOe between Ha and He will increase, and a point may come when it totally cancels the larger, indirect negative nOe effect exerted by Ha on He through Hb. Thus no nOe may be observed at He upon irradiation of Ha, even though Ha and He are spatially close The absence of nOe between nuclei therefore does not necessarily mean they are far from one another. 

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. 

In a system, when 0 = 180°, the distance between spins A and C (tac) will be a maximum and the nOe between A and C a mimimum. When 9 decreases from 180°, r c decreases and the direct nOe between A and C increases. As a result of the consequently more effective A-C relaxation, the B-C relaxation process becomes relatively less important, and the indirect negative A-C contribution is correspondingly decreased. When 0 = 78°, r c = 1.26, so there will be no net nOe between A and C, even though the A and C spins are very close to each other. This is because the direct (positive) and indirect (negative) nOe effects are equal and opposite. 

The NOE is really quite complicated, and in fact even small molecules can show negative NOEs, which are due to a phenomenon known as spin diffusion. 

Problem 41 confronts you with a phenomenon which we have referred to briefly in Section 1.1.6 a negative NOE signal. The point to note is that this is real do not worry about how it arises ... 

This suppression scheme has been shown to work well together with HMQC experiments of small molecules at natural abundance. Even cleaner spectra are obtained, if the BIRD sequence is combined with HSQC experiments already containing a spin-lock purge pulse. Drawbacks of the BIRD pulse scheme are the fact that the relaxation delay between scans cannot be chosen freely anymore and that complete suppression of all C-bound proton signals is impossible, if they have different relaxation times. Furthermore, the BIRD pulse scheme is not applicable to molecules in the slow motional regime, since negative NOEs between the inverted proton spins and the non-inverted C-bound proton spins would reduce the magnetization of the latter. 

Kondo, T. et al.. Heavenly blue anthocyanin. IV. Structure determination of heavenly blue anthocyanin, a complex monomeric anthocyanin from the morning glory Ipomoea tricolor, by means of the negative NOE method. Tetrahedron Lett., 28, 2273, 1987. 

Quantitative evaluation of NOE magnitudes in terms of internuclear distance is problematic and often questionable, since there are influences (experimental and structural) in addition to distance. In particular, if a third NMR active nucleus X is close to nucleus I, reduced enhancements of the 1 signal may be observed because the dipolar cross relaxation of I is dominated by X and no longer by the irradiated S. Indeed, negative NOEs (signal intensity decreases) may occur212. 

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

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