NOE Difference Spectra

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.

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. 

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). 

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... 

In the nOe difference spectrum, only the nOe effects of interest remain, while the unaffected signals are removed by subtraction. It does not therefore matter if the nOe responses are small or buried under the unaffected signals, since they show up in the difference spectrum. The main benefit of nOe difference spectroscopy is that it converts the changes in intensity into a form that is more readily recognizable. The difference spectrum is obtained by a process in which a control (normal) spectrum is subtracted from a spectrum acquired with irradiation of a particular signal. 

Figure 6. Xenognosin A nOe difference spectrum. (360 MHZ, acetone-dg). Irradiation of H2 gives the enhancements shown in the upper trace.

Figure 12b shows such an NOE difference spectrum for the acetal 3 the spectrum was obtained by irradiating the methine doublet at about 5.8 ppm (the normal spectrum of 3 is shown in Fig. 12a). 

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... 

FIGURE 17. H NMR spectra of 73d (ca 25 mgmR1 CDCI3) (a) olefinic region and (b) NOE difference spectrum (irradiation at 2.06 ppm). Reproduced by permission of Neue Schweizerische Chemische Gesellschaft from Reference 43... 

Fig. 3. (a) 300 MHz proton spectrum of the molecule shown, and (b) 500-fold expanded NOE difference spectrum obtained using reference deconvolution to enforce identical reference lineshapes (2 Hz Lorentzian) for the chloroform signals in the irradiated and unirradiated spectra. Note the complete absence of the usual up-down difference artefacts in trace (b), even for the sharp singlet methyl signals the remaining artefacts are largely homodecoupler... 

Figure 7. H-NMR spectra of vulgarin (a) normal ID H spectrum, (b) NOE difference spectrum with the signal at d = 4.13 (H-6) irradiated (taken with permission from ref 14).

Load the reference spectrum D NI /IRDATA GLUCOSE 1D H GHNO 1D 008999.IR from the NOE data for glucose. From the Display pull-down menu choose the Dual Display option and select the NOE-difference spectrum 007999.1 R in the same directory. 

The upheld resonance at — 1.98 ppm from DSS (or — 6.71 ppm from HDO) was assigned to the 72-CH3 of 367(Ell)Val (Lindstrom et al., 1972b). The NOE difference spectrum with preirradiation on this resonance for 100 msec is shown in Fig. 19B (b). The observed spectral features are very similar to those shown in Fig. 19A (b). Following the same procedures and considerations used for the a chain, we have assigned the resonances at —0.10 and +1.36 ppm from DSS... 

Figure 2.8. H NMR NOE difference spectrum of the colored open form obtained upon irradiation of the olefinic proton H a.

E3-3 Shown below are a the 250 MHz >H spectrum of this pyridine derivative and b the NOE difference spectrum resulting from pre-irradiation of Hy. What conclusions can you draw about the stereochemistry, conformation and bond rotations of this molecule ... 

A large enhancement of H2 from H3 would not have been expected for conformation a because H2 is much closer to Hy. So there must also be a large population of conformation b. We cannot easily quantify the relative populations from this NOE difference spectrum because H2 and H4 are in such different proton environments ( 6.5.3). Equilibration between the two conformations is obviously fast on the chemical shift timescale. The appropriate experiment here is separate irradiation of H2 and H4 and measurement of the relative enhancements at H3 and H . This experiment is illustrated below, and the resulting spectra are shown on the next page. 

The two amide NH signals were equally enhanced in the first NOE difference spectrum (b, p. 64). The presence of two signals shows that they are not exchanging rapidly on the chemical shift timescale and structure c shows that they would be expected to be different distances ... 

A reaction product is known to have one of the two structures shown, which differ only in whether the carbonyl is south and the epoxide is north or vice versa. Use the spectra below to determine which is corrrect. Trace a is a 13C spectrum acquired without any proton decoupling at any stage. Trace b is the 13C NOE difference spectrum resulting from selective pre-irradiation of the non-aromatic single proton. 

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