Correlation spectroscopy, COSY interpreting

Homonuclear NMR Structural elucidation becomes much more difficult as spectral complexity increases. Under these circumstances, proton NMR spectroscopy has benefited considerably from the use of 2D NMR techniques. For example, homonuclear correlation spectroscopy (COSY) identifies spin-coupled pairs of nuclei as well as spin-coupled networks of nuclei in a molecule, even without prior structural information. The 2D /-resolved spectroscopy method permits even highly overlapping resonances to be resolved into readily interpretable multiplets. This enables chemical shift assignments to be made in a very straightforward manner. Both of... 

ACF = atom-centered fragment AI = artificial intelligence BAM = bond adjacency matrix CHEMICS = combined handling elucidation method for interpretable chemical structures COSY = correlated spectroscopy FBMX = free-bond connection matrix HMBC = heteronuclear multiple bond correlation spectroscopy HMQC = heteronuclear multiple quantum coherence correlation spectroscopy SESAMI = systematic elucidation of structure applying machine intelligence. 

In the case of an unknown chemical, or where resonance overlap occurs, it may be necessary to call upon the full arsenal of NMR methods. To confirm a heteronuclear coupling, the normal H NMR spectrum is compared with 1H 19F and/or XH 31 P NMR spectra. After this, and, in particular, where a strong background is present, the various 2-D NMR spectra are recorded. Homonuclear chemical shift correlation experiments such as COSY and TOCSY (or some of their variants) provide information on coupled protons, even networks of protons (1), while the inverse detected heteronuclear correlation experiments such as HMQC and HMQC/TOCSY provide similar information but only for protons coupling to heteronuclei, for example, the pairs 1H-31P and - C. Although interpretation of these data provides abundant information on the molecular structure, the results obtained with other analytical or spectrometric techniques must be taken into account as well. The various methods of MS and gas chromatography/Fourier transform infrared (GC/FTIR) spectroscopy supply complementary information to fully resolve or confirm the structure. Unambiguous identification of an unknown chemical requires consistent results from all spectrometric techniques employed. 

It was stated in the opening remarks of this chapter that COSY was the first two-dimensional sequence proposed, and it is in fact given in Fig. 5.2, utilising just two 90° pulses. The sequence, which correlates the chemical shifts of spins that share a mutual /-coupling, is most often applied in proton spectroscopy although it is equally applicable to any high-abundance nuclide. It is without doubt the mostly widely used of all two-dimensional methods and is thus considered first. This section provides an introduction to the operation of the experiment and its principal features, after which you should be in a position to understand and interpret routine COSY spectra. Later sections look at more specific and useful features of the basic experiment and those of its... 

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