Homonuclear NOE experiments

In a heteronuclear nOe experiment, the first step may be identical to the homonuclear nOe experiment (i.e., involving an incoherent transfer of coherence from H to H/), while the second step could involve a coherence transfer from H to C nucleus by an INEPT sequence. These methods suffer from poor sensitivity and have therefore not been used extensively. 

We have concentrated on the proton-proton, homonuclear NOE experiments in this section but the potential use of analogous heteronuclear experiments should not be overlooked, if you have the appropriate hardware available to you. The 19F- 1H NOE experiment, for example, can be very useful in certain situations as demonstrated in the following example. You have one of two possible positional isomers (Structure 8.6). 

EXAMPLE 12.7 What is the maximum NOE signal enhancement expected in a homonuclear NOE experiment ... 

NOE is widely used in three-dimensional structure determination of molecules, especially for proteins in solution. NOE-based NMR experiments provide an effective means to investigate nanostructural organizations in ionic Hquids. Homonuclear NOE experiments (NOESY) and heter-onuclear NOE experiments (HOESY) have been extensively appfred to probe intra- and intermolecular interactions within ILs and interaction of ILs with solvents and inorganic salts. Due to the abihty to probe dipolar coupled through space interactions, NOE is a perfect choice to probe interionic interactions to understand ion-pair dynamics. Since quantum chemical calculations are a convenient method to study ion-pair interactions and spatial information about them [76-78], combinations of this with NOE experiments would be a powerful approach to understand site-specific ion-pair interactions. 

Heteronuclear nOe experiments are similar to homonuclear experiments, discussed earlier. Normally, protons are irradiated and the enhancement of the heteronucleus is recorded, since if the reverse were done, the... 

Both homonuclear and heteronuclear versions of relayed nOe experiments are known. The homonuclear relayed NOESY experiment involves both an incoherent transfer of magnetization between two spins H and H/ that are not coupled but close in space, and a coherent transfer of magnetization between two spins H(and H that are /-coupled together. The magnetization pathway may be depicted as... 

We now want to turn to another experiment which, we must make clear at the start, does not have any relationship in theory to NOE experiments. In fact the theory is so complicated that we shall not say anything about it at all, but just refer you to one of the books in the Appendix. We are including this experiment because of its unique advantages when the spectrum has overlapping multiplets. It is called TOCSY, which stands for Total Correlation SpectroscopY (it has a second, more amusing name HOHAHA, standing for HOmonuclear HArtmann-HAhn), and is of particular use when oligosaccharides or peptides are under study. 

The ID NOE experiment is not restricted to the homonuclear H/H-ease, but may also be performed in a heteronuclear mode, i.e. by observing NOEs for C nuclei induced by selectively preirradiated protons. 

All of the protons in each of 12 thermo- and photochromic BIPS were assigned through a combination of homonuclear decoupling experiments and correlation spectroscopy. The relative stereochemistry of the gem-dimethyl groups could be assigned on the basis of Nuclear Overhauser Effect (NOE) experiments.131... 

For the homonuclear proton NOE experiment ( H H ) that parallels experiments (a) and (b) in Figure 5-11, it has traditionally been supposed that the NOE (in percentage, IOOt]) must exceed about 5% to be accepted as experimentally significant. The difference NOE experiment, however, can measure enhancements reliably to below 1%. In this procedure, spectra obtained by the methods analogous to Figure 5-1 la and b are alternatively recorded and subtracted. Unaffected resonances disappear, and NOE s are signified by residual peaks. 

Nuclei that undergo mutual relaxation via dipolar coupling are said to be dipolar coupled and give rise to the nuclear Overhauser effect. Whether the nuclei in question also may be scalar (or spin) coupled is not pertinent to the discussion (Section 5-4). NOE experiments can be either homonuclear or heteronuclear in nature, although the former, involving protons, are much more common. One-dimensional homonuclear Overhauser experiments are discussed in Section 7-3 their 2D versions, NOESY and ROESY, are treated in this section. 

For APT, COSY and C/ H HETCOR experiments, standard pidses sequence were used. (9-11) For the long-range HETCOR experiment, J(C,C,H) was assumed to be 8 Hz. NOE Difference spectra (homonuclear decoupling experiments ) were recorded using the cyclenoe sequence. 1024 scans were recorded for each spectrum. 

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