Difference NOE

Figure Bl.11.14. NOE-difFerence spectrum (see the text) of aspirin, with pre-saturation at the methyl resonance, proving that the right-hand confomration is dommant.

NOE-difFerence spectroscopy is particularly valuable for distinguishing stereoisomers, for it relies solely on intemuclear distances, and thus avoids any problems of ambiguity or absence associated with couplings. With smallish molecules, it is best carried out in the above 1D maimer, because 2 s are necessary for tire transmission of the NOE. The transmission process becomes more efficient with large molecules and is almost optimal for proteins. However, problems can occur with molecules of intemiediate size [3f]. A 2D version of the NOE-difference experiment exists, called NOESY. 

NOE difference spectroscopy is used to identify short, throughspace interactions and measure these interactions to an upper limit of about... 

Nuclear Overhauser enhancement (NOE) spectroscopy has been used to measure the through-space interaction between protons at and the protons associated with the substituents at N (20). The method is also useful for distinguishing between isomers with different groups at and C. Reference 21 contains the chemical shifts and coupling constants of a considerable number of pyrazoles with substituents at N and C. NOE difference spectroscopy ( H) has been employed to differentiate between the two regioisomers [153076 5-0] (14) and [153076 6-1] (15) (22). N-nmr spectroscopy also has some utility in the field of pyrazoles and derivatives. 

Substituent effects (electronegativity, configuration) influence these coupling constants in four-, five- and seven-membered ring systems, sometimes reversing the cis-tmns relationship so that other NMR methods of structure elucidation, e.g. NOE difference spectra (see Section 2.3.5), are needed to provide conclusive results. However, the coupling constants of vicinal protons in cyclohexane and its heterocyclic analogues (pyranoses, piperidines) and also in alkenes (Table 2.10) are particularly informative. 

Figure 2.21. HFI NOE difference spectra (b, c) and FIFI NOESY diagram (d) of a-pinene (1) with /-/ NMR spectrum (a) for comparison [(CD3)2CO, 10% v/v, 25 °C, 200 MHz, section from <5 = 0.85 to 2.34 ]. Vertical arrows in (b) and (c) indicate the irradiation frequencies in the HH NOESY plot (d), cross-signals linked by a dotted line show the NOE detected in (c)...

A comparison of the methods of proton-proton NOE detection has shown that two-dimensional NOE detection such as NOESY and ROESY are better suited to the investigation of the stereochemistry of biopolymers whereas for small- to medium-sized molecules (up to 30 C atoms) NOE difference spectroscopy is less time consuming, more selective and thus more conclusive. 

Jhh coupling constants HH NOE difference spectra HHNOESY or HHROESY... 

Several shifts and coupling eonstants in the NMR speetra set 34 are so typical that the carbon skeleton can be deduced without any additional information. An NOE difference spectrum gives the relative configuration of the eompound. 

Conditions CDCI3, 25°C, 200 MHz ( //), 50 MHz ( C). (a) NMR spectrum with expanded multiplets (b) NOE difference spectrum, irradiated at Sff = 1.87, (c) C NMR partial spectra, each with H broadband decoupled spectrum below and NOE enhanced coupled speetrum (gated decoupling) above (d) CH COSY diagram ( empty shift ranges omitted). 

The hydrochloride of a natural product which is intoxicating and addictive produced the set of NMR results 53. What is the structure of the material What additional information can be derived from the NOE difference spectrum ... 

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. 

Fragments E and F which include all 16 carbon atoms detected by C NMR can be attached to each other in two ways G or H the structure G is realised, as follows from the NOE difference spectra, which show a significant NOE between the methyl protons at Sh = 1-28 and the alkene proton at Sh = 6.28 and vice versa (Table 44.3). 

Table 44.3. Significant proton-proton NOEs from the HH NOE difference spectra...

Further information is derived from the NOE difference spectra with decoupling of the methyl protons. Table 51.2 summarises the most significant NOE enhancements to complete the picture. 

The relative configuration is derived from the NOE difference spectra. Significant NOEs are detected owing to cis relationships within the neighbourhood of non-geminal protons ... 

Structure and stereochemistry of the dibromides 2a-h were elucidated by various NMR methods including ID proton-proton NOE difference spectra and 2D... 

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... 

The nOe difference spectrum has the advantage that it allows measurements of small nOe effects, even 1% or below. The experiment involves switching on the decoupler to allow the buildup of nOe. It is then switched off, and a w/2 pulse is applied before acquisition. The nOe is not affected much by the decoupler s being off during acquisition, since the nOes do not disappear instantaneously (the system takes several Ti seconds to return to its equilibrium state). 

Explain the main advantage of nOe difference spectroscopy. Why does it involve a mathematical subtraction of the normal H-NMR spectrum from the nOe-enhanced H-NMR spectrum ... 

It is important that constant temperature be maintained throughout the nOe difference experiment. If the instrument is fitted with a constant-temperature device, then it is advisable to adjust it to a few degrees above room temperature so that it maintains a constant temperature accurately. 

The nOe difference spectrum is highly demanding, since even the slightest variation in the spectra recorded with and without preirradiations will show up as artifacts in the difference spectrum (Fig. 4.8). The errors can be random, due to phase instability caused by temperature effects on the Rf circuits, variations in spinner speed, etc. The problem of phase instability is reduced in the latest generation of instruments with digital... 

Avoid having moving metal objects near the magnet when carrying out nOe difference experiments, to prevent random variations in frequency. A small line-broadening ( 2 Hz) can also be applied to the spectra before or after subtraction, to reduce subtraction artifacts. 

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. 

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