Chemical shifts multiple bonds, sensitivity

The multiplicity of these resonances provides direct evidence for the number of directly bonded protons, and the magnitudes of the couplings reflect the orientation of the associated substitutent, at least at the anomeric centre (13). Now, although these spectra are rather impressive - indeed we believe that they are the first 13c spectra of a sugar to be measured at 90.5 MHz - it is important to stress that, even had this spectrum been measured at 20 MHz, the dispersion between the individual transitions would have been adequate for our purposes. This is because 13c chemical shifts are extremely sensitive to their chemical and stereochemical environment. Thus, for derivative (A) the entire proton spectrum covers a spectral region oI only ca. 3.5 ppm whereas the i3c spectrum spans ca. 70 ppm - an approximately twenty fold increase. 

Notably, two isomeric products can be generated. The usual infrared (IR) and mass spectra as well as H and 13C NMR chemical shifts could not define which isomer was formed. The authors used different NMR techniques, such as 2-D heteronuclear multiple bond correlation (HMBC) experiments and phase-sensitive nuclear overhauser enhancement spectroscopy (NOESY) measurements to elucidate the product s structure. 

Bax A, Davis DG (1985) MLEV-17 Based two-dimensional homonuclear magnetization transfer spectroscopy. J Magn Reson 65 355-360 Bax A, Drobny G (1985) Optimization of two-dimensional homonuclear relayed coherence transfer NMR spectroscopy. J Magn Reson 61 306-320 Bax A, Marion D (1988) Improved resolution and sensitivity in H-detected heteronuclear multiple-bond correlation spectroscopy. J Magn Reson 78 186-191 Bax A, Subramanian S (1986) Sensitivity-enhanced two-dimensional heteronuclear chemical shift correlation NMR spectroscopy. J Magn Reson 67 565-569 Bax A, Summers MF (1986) and Assignments from sensitivity-enhanced detection of heteronuclear multiple bond connectivity by 2D multiple-quantum NMR. J Am Chem Soc 108 2093-2094... 

But Rh, Ag, or Pt exhibit chemical shift differences - preferentially determined by heteronuclear multiple bond coherence (HMBC) spectra-as well as J-coupling in the form of satellites in the spectra of the sensitive nuclei [12]. However, some paramagnetic nuclei increase the magnetic relaxation and increase the line broadening. 

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