The advantages of the indirect detection
The reverse detection technique became increasingly popular due to the software improvements of the spectrometers from one hand and to the increase of the sensitivity of the detection of a X nucleus by the indirect way. In fact the reverse detection solves the problem of the low concentrations. Moreover, it allows to reach the NMR parameters such as chemical shifts, coupling constants and relaxation time spin lattice for nuclei impossible to study by the direct detection (Fig. 28).
For example it is not always obvious to get NMR spectra for nuclei such as 57Fe, 183W, 187Os and sometimes even 15N in direct observation. Consequently, the reverse detection becomes the only possible way either for 1H-15N correlation in the case of peptides or proteins or also for 1H-X correlation in the case of organometallic compounds. In each of these examples the reverse detection allowed to determine the chemical shift of the heteronucleus together with the coupling constants J(X-H).
The arrows figure the magnetization transfer from the proton towards the 13C and then from the 13C towards the proton.
The available pulse sequences
The HSQC (Heteronuclear Single Quantum Coherence) is specially used for the structural determination of small proteins. The HMBC (Heteronuclear Multiple Bond Coherence) is optimized for long range couplings and works better than a COLOC.
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