Coupled Spectrum Gated Decoupling

It is in fact quite simple to record a carbon-13 spectrum with the broadband decoupling switched off. Such a procedure has the disadvantage that the gain in signal intensity due to the NOE is lost, so that measurement times are very long. 

Because of the NOE and differences in relaxation rates, the intensity differences for carbon signals in a broad-band decoupled spectrum are extremely large, so that quantitative information is not available. 

Though this is generally not a problem, there is an experiment available which allows us to obtain reliable quantitative intensity information, which we may for example need when studying mixtures of compounds. 

However, in the inverse gated experiment it is very important that the relaxation delay chosen is very long, since the carbon atoms have very different relaxation times (and relax by different mechanisms). In our example the relaxation time was set to 120 seconds This of course makes the experiment a very time-consuming one (28 hours measurement time ). 

The integration of the various carbon signals now gives intensity values which are sufficiently accurate for most purposes. 

In the carbon-13 experiments so far discussed, only a single radio-frequency pulse has been used to irradiate the spin system. This gave us information on the chemical shifts of the carbon nuclei in the molecule. The coupled spectrum obtained using gated decoupling (1.2.2) told us how many protons are bound to any one carbon atom however, this experiment requires a lot of time. There are however other experiments which give us this information 

Both find their origin in the spin-echo sequence, devised hy Hahn in 1952 and used for the determination of relaxation times. 

The theory behind both of these experiments, and in particular the DEPT experiment, is rather complicated, so that we refer you to NMR textbooks for details. The important feature of both is that the carbon signals appear to have been simply broad-band decoupled, but that according to the multiplicity they appear either in positive (normal) phase or in negative phase, according to their multiplicity. 

There is another member of this family of experiments known as INEPT (Insensitive Nuclei Enhancement by Polarization Transfer), which was the forerunner of DEPT. INEPT still has its uses for obtaining spectra of really insensitive nuclei such as silicon-29 or nitrogen-15. 

The information on carbon chemical shifts and multiplicities is invaluable for structure determination. It would be ideal if we also had a method for obtaining information directly on carbon-carbon bonding in the compound under study, since this would allow us to draw on paper at least parts of the carbon framework of the molecule. 

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No further information is required to identify this compound from its NMR spectra 15. Conditions CDCI3, 25 °C, 20 MHz. (a) Proton broadband decoupled spectrum (b) NOE enhanced coupled spectrum (gated decoupling) (c) expanded section of (b). 

Conditions (003)280, 25 °C, 400 and 600 MHz ( //), 100 MHz ( C). (a) HH COSY plot (600 MHz) following D2O exchange (b) H NMR spectra before and after deuterium exchange (c) sections of the C NMR spectra, in each case with the H broadband decoupled spectrum below and NOE enhanced coupled spectrum (gated decoupled) above ... 

Conditions CDCI3, 25°C, 200 MHz (JH), 50 MIIz (l3C). (a) ll NMR spectrum with expanded multiplets (b) NOE difference spectrum, irradiated at SH = 1.87 (c) 13C NMR partial spectra, each with H broadband decoupled spectrum below and NOE enhanced coupled spectrum (gated decoupling) above (d) CH COSY diagram ( empty shift ranges omitted). 

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