Homonuclear spectral editing

MQMAS has further opened up vistas by making it possible to design high-resolution hetero- and homonuclear correlation experiments. These lead to connectivity information, distance measurements, and spectral editing. 

In Section 9.3.1, the problem of assigning the many resolved resonances in a ID MAS spectrum was mentioned, and ID spectral editing methods were introduced. In this section, we describe homonuclear 2D correlation experi-... 

Figure 3.25 Homonuclear decoupling experiment. A 250 MHz H NMR of ethylbenzene in deuterated chio-rotorm obtained (a) without decoupling, (b) with irradiation of the methyl resonance, and (c) with irradiation of the methylene resonance. (From Bruch, M.D. and Dybowski, C., Spectral editing methods for structure elucidation, in Bruch, M.D., ed., NMR Spectroscopy Techniques, 2nd edn., Marcel Dekker, Inc., New York, 1996. Used with permission.)...

Clearly the homonuclear and the heteronuclear experiments could be combined in the reverse order i.e. HSQC-NOESY and HSQC-TOCSY. The main advantage in these schemes relates to N-edited experiments in which the narrower amide proton spectral width is sampled during f and the full proton spectral width is collected during t. On the other hand water suppression is more effective when HSQC follows NOESY. Also the sensitivity enhancement can be incorporated into the NOESY-HSQC experiment. Eor the TOCSY-HSQC or HSQC-TOCSY it does not matter because both the TOCSY and HSQC sequences can be implemented with the sensitivity enhancement. It should be mentioned that the TOCSY type of transfer is more effective between C nuclei than between protons and therefore the HCCH-TOCSY experiment is preferred when a doubly labeled sample is available. 

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