Enhancement, NOE

Figure 1.10. NMR spectra of 2,4,6-trichloropyrimidine [CeDe, 75% v/v 25 °C, 20 MHz], (a) NMR spectrum without proton decoupling (b) NOE enhanced coupled NMR spectrum (gated decoupling)...

Conditions CDCI3, 25 °C, 20 MHz. (a) //broadband decoupled spectrum (b) expanded sp shift range (c) expanded sp shift range (b) and (c) each with the H broadband decoupled spectrum below and NOE enhanced coupled spectrum above. 

From which compound were the INADEQUATE contour plot and C NMR spectra 21 obtained Conditions (CD3)2CO, 95 % v/v, 25 °C, 100 MHz. (a) Symmetrised INADEQUATE contour plot with C NMR spectra (b) H broadband decoupled spectrum (c) NOE enhanced coupled spectrum (gated decoupling) (d) expansion of multiplets of (c). 

Conditions CDCI3, 25 °C, 100 MHz ( C), 400 MHz H). (a-e) C NMR spectra (a,b) //broadband decoupled spectra (c,d) NOE enhanced coupled spectra (gated decoupling) with expansion (e) of the multiplets in the sp shift range (f) //NMR spectrum with expanded multiplets. 

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). 

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. 

NOE enhancements (Table 51.2) reflect coaxial relationships between... 

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 ... 

When the zero-quantum 1% transition is greater than double-quantum Wi, the nOe enhancements will be negative. Similarly, when is greater than 1%, the resultant nOe will have a positive sign. The predominance of Wi and over one another depends on the molecular motion. It is known that the Wo transition is maximal when the molecule tumbles at a rate of about 1 KHz, while the Wi transition is fastest at a tumbling rate of about 800 MHz. On this basis, a rough idea of the sign of nOe can be obtained. For example, small molecules in nonviscous... 

NMR provides one of the most powerful techniques for identification of unknown compounds based on high-resolution proton spectra (chemical shift type integration relative numbers) or 13C information (number of nonequivalent carbon atoms types of carbon number of protons at each C atom). Structural information may be obtained in subsequent steps from chemical shifts in single-pulse NMR experiments, homo- and heteronuclear spin-spin connectivities and corresponding coupling constants, from relaxation data such as NOEs, 7) s 7is, or from even more sophisticated 2D techniques. In most cases the presence of a NOE enhancement is all that is required to establish the stereochemistry at a particular centre [167]. For a proper description of the microstructure of a macromolecule NMR spectroscopy has now overtaken IR spectroscopy as the analytical tool in general use. 

If you do manage to get everything right, NMR offers excellent quantisation results. What s more is that it is free if you have acquired a 1-D spectrum. Note that you can use this approach to quantify other nuclei - it works just as well for 19F. Note that it won t work very well for 13C because we normally acquire 13C data with NOE enhancement from the protons so the signals are not quantitative. (It is possible to collect carbon data in a quantitative way but it is not something that we would normally do...). 

Fig. 10.7. DPFGSE NOE of H5 of antillatoxin (3) showing good selection and clean NOE enhancements.

Broadband XH decoupling, in which the entire proton spectral window is irradiated, collapses all of the 13C multiplets to singlets, vastly simplifying the 13C spectrum. An added benefit of broadband proton decoupling is NOE enhancement of protonated 13C signals by as much as a factor of three. 

The first NOE-enhanced 15N image which was reported127 was for aniline-15N. It is interesting, because in this sample the exchange processes cause partial collapse of the ... 

Nuclear Overhauser effect—The nuclear Overhauser effect (NOE) occurs only between nuclei that share a dipole coupling, i.e., their nuclei are so close that their magnetic dipoles interact. Techniques that use NOE enhance spectra and allow spacial relationships of protons to be determined. 

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