NOE experiment

A disadvantage of DEPT is that it is a subtraction experiment and, therefore, much more sensitive to certain problems than are typical one-dimensional techniques. One remedy that is used in many experiments that suffer fi om stability problems is the employment of steady-state, or dummy, scans (parameter 2 in the foregoing list Section 2-4i). Poor signal cancelation is generally the result of difficulties in one or more of the following areas  

lock stability keep the lock power just below saturation and the lock gain in the 30% range. 

temperature of the sample perform experiments at a constant temperature. 

incomplete cancelation in spectral subtractions steady-state scans are critical. 

A modified DEPT experiment, DEPTQ, recently was developed that permits the detection and display of nonprotonated nuclei, such as quaternary carbons, and that possesses the editing features and polarization transfer sensitivity advantages of the DEPT sequence. The DEPTQ experiment promises to become an important method for the determination molecular structures. 

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The most widely used nOe experiment is nOe difference spectroscopy. Two different sets of experiments are recorded, one in which certain protons are subjected to irradiation and enhancements are obtained of other... 

Why is degassing of sample solutions in NMR tubes essential before nOe experiments, and why are aqueous solutions (solutions in DgO) not generally degassed in the NMR tubes ... 

What precautions are normally taken during sample preparation for the nOe experiment ... 

The nOe experiment is one of the most powerful and widely exploited methods for structure determination. nOe difference (NOED) or the two-dimensional experiment, NOESY, is used extensively for stereochemical assignments. It provides an indirect way to extract information about internuclear distances. The other use of nOe is in signal intensification in certain NMR experiments, such as the broad-band decoupled C-NMR experiment. 

A large number of ID nOe experiments may have to be performed if the spatial relationships among many protons in a molecule are to be determined. In such cases, instead of employing multi pulse ID nOe experiments, we can opt for the nOe spectroscopy (NOESY) experiment. If many protons have close chemical shifts, then NOESY may be particularly advanta-... 

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... 

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. 

Cross-relaxation The mutual intermolecular or intramolecular relaxation of magnetically equivalent nuclei, e.g., through dipolar relaxation. This forms the basis of nOe experiments. 

The oldest and most widely used structural restraints in NMR spectroscopy are distance restraints derived from NOE experiments [1]. Transient NOE, 2D NOESY and ROESY spectra provide valuable information for interatomic distances up to 5 A that will be discussed in the following. 

Fig. 9.2 Schematic representation of the three basic experiments useful for the determination of (A) transient NOE experiment, (B) 2D NOESY and (C) 2D ROESY. The gray-filled half-circle represents a frequency-selective inversion pulse which inverts the spin to which the cross-relaxation...

A single C-O adduct 36 is formed in the reaction this adduct is not converted to the desired product. Coupling constants and the results of an nOe experiment... 

The formation of such complexes apparently involves a delicate balance of binding forces, since a-phenyl-ethylamine 30 shows only modest tendencies to form 2 1 complexes and its stacking efficiency is reduced. The structural details of these complexes are not known, but intermoleeular NOE experiments favor structures such as shown in 31. The distance between the aromatic and amine recognition sites in the... 

These remarks only apply to the proton-proton NOE experiments involving an NOE between the proton and another nucleus can also be carried out, and the NOE also has an effect on certain carbon-13 spectra, as we shall see later. 

There are two-dimensional NOE experiments (see below, Section 2.3), but first we shall consider the one-dimensional measurements, which are of two types. To make these clear we shall use molecules 1 and 3. 

Fig. 12a—c NOE experiments carried out at 200 MHz on compound 3. a Normal spectrum, with expansion of methine doublet b selective NOE spectrum, total time required 18 min c NOE difference spectrum, total time required (preparation, measurement) 42 min... 

Advances in computer and spectrometer design have made possible an NOE experiment which does not rely on spectrum subtraction. This is some-... 

You may wonder why we did not use our model compound 1 in order to demonstrate the NOE. The reason becomes quite clear when we look at the result of a selective NOE experiment carried out at 600 MHz on 1, which is shown in Fig. 13. 

We have seen that NOE experiments are very useful and can give information on relative interproton distances in the molecule. However, we should stress that NOE experiments can be difficult to interpret because of the many factors involved in their generation. 

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 previous experiment (COSY) demonstrated the interactions (J coupling) between protons via the bonding electrons. The NOE effect which we described in Section 1.1.6 functions because of the through-space interactions between protons, and we used the NOE difference and selective NOE experiments to demonstrate it. 

The advantage of the 2D NOE experiment over selective ID NOE measurements is that all the NOEs present in a compound are detected in one single... 

As we have mentioned already, a very useful tool when trying to identify exchangeables is to exchange them for deuterium, which removes them from the spectrum. This will be covered in detail in Chapter 7 but don t be in too much of a hurry to do this - they are part of the spectrum and hold valuable information. If they are sharp enough, they may become potentially useful targets for NOE experiments which we will discuss later. 

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