Decoupling NOESY

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. 

Figure 7.13 Pulse sequences of semisoft NOESY. (a) Normal (b) same but with additional Fi decoupling. (Reprinted from Mag. Reson. Chem. 29, H. Kessler et al., 527, copyright (1991), with permission from John Wiley and Sons Limited, Baffins Lane, Chichester, Sussex P019 lUD, England.)...

Whereas spin decoupling, COSY and TOCSY techniques are used to establish connectivities between protons through bonds, techniques that make use of the nuclear Overhauser effect (NOE), such as 1-D NOE and NOESY, 1- and 2-D GOESY, 1- and 2-D ROESY, can establish connectivities through space. Before looking at these techniques in detail, it s worth spending a little time considering the NOE phenomenon itself - in a nonmathematical manner, of course ... 

In the case of isoorientin 6"-0-caffeate isolated from Gentiana arisanensis, the C NMR spectrum was assigned by H-decoupled spectra, DEPT pulse sequence, H- C COSY spectrum, long-range C- H COSY, and NOESY experiments the H NMR spectrum was analyzed with the aid of H- H COSY and H- C COSY. 

Fig. 2.19. HH NOE difference spectra (b, c) and HH NOESY diagram (d) of a-pinene (1) with H NMR spectrum (a) for comparison [(003)200, 10% v/v, 25 °O, 200 MHz, section from 0.85 to 2.34 ppm)]. Vertical arrows in (b) and (c) indicate the decoupling frequencies in the HH NOESY plot (d), cross-signals linked by a dotted line show the NOE detected in (c)...

Normal H-NMR spectrum Decoupled C-NMR spectrum COSY spectrum HETCOR or HMQC spectrum COLOC or HMBC spectrum NOESY spectrum or ID nOe difference spectra. 

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