Proton decoupling inverse gated

Keto-enol tautomerism of fi-keto esters can be analyzed by carbon-13 NMR as illustrated by an inverse gated proton-decoupled 13C NMR spectrum of ethyl acetoacetate in Fig. 4.6 [73 i]. 

FIGURE 4.4 (a) Inversion recovery pulse sequence with inverse gated proton decoupling for 7, measurement. 

FIGURE 4.7 Inverse gated proton decoupling pulse sequence. Rd is relaxation delay, 0 is a variable pulse angle, t2 is the acquisition time. 

Because the application of proton broad band decoupling in solution N NMR causes quantification problems, the inverse-gated decoupling technique often is used. With this technique no enhancement is observed, and the spectra can be quantified. However, a problem that arises in NMR experiments with nuclei with low sensitivity such as N is acoustic ringing. Ringing occurs because the rf pulse causes brief vibrations in the probe which masquerade as a signal. This signal overlaps the entire spectra which results in a distortion of the baselines. Quantification of these distorted spectra is impossible and in many cases the spectra become useless. 

Figure 4,9. Possible schemes for applying proton decoupling when observing a heteronucleus, (a) Gated decoupling (coupled spectrum with NOE), (b) inverse-gated decoupling (decoupled spectrum without NOE) and (c) power-gated decoupling where the rf is applied at two different powers (decoupled spectrum with NOE).

FIGURE 5.7. Inverse gated broadband proton decoupling pulse sequence. See Figure 5.5 for comparison. 

For proton NMR the main consideration is ensuring complete relaxation between successive pulses for all the different types of hydrogen atoms present. This requires the interpulse delay to be at least 5 times the longest Tj. In addition, for nuclei such as C. where broadband decoupling is usually required, the inverse gated technique (Section 18.3.4.2) should be used to prevent the occurrence of NOE effects. A further consideration in the case of spectra from nuclei such as C and F, which may have very wide spectral widths, is whether the RF pulse has sufficient power to irradiate all the nuclei equally effectively. The digital resolution and data processing requirements for a particular application also require careful selection. 

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