Interrupted proton decoupling

Interrupted-Proton-Decoupling Experiments. Interrupted-pro-ton-decoupling experiments were carried out with a 50- xs interruption. This technique selects quaternary carbons (those lacking an attached proton) and rapidly moving methyl carbons. Resonances from methylene and methine carbons are suppressed unless, for some reason, their motion within the solid occurs at a rate similar to that of methyl groups. In essence, the interrupted-proton-decoupling experiment, which has not been previously reported for amber samples, provides an alternative fingerprint for the samples. 

Figure 3 gives the results of interrupted proton decoupling for the Simojovel sample with the 13C NMR spectrum shown in Figure IB and for the Baltic sample with the 13C NMR spectrum shown in Figure 2A. 

Figure 3. The 13C NMR spectra of Mexican amber from Simojovel and Baltic amber taken with interrupted proton decoupling.

The pulse sequences used for this purpose are known as interrupted decoupling (Gerasimowicz et al. 1984, Haw et al. 1984, Barron et al. 1985, Maciel et al. 1985), delayed decoupling (Manders 1987) or dipolar dephasing (Hatcher 1987). All involve an interval, t inserted in the pulse sequence between the end of the contact time and the beginning of data acquisition as shown in Fig. 4.5.2. Both nC and proton RF transmitters are switched off for the duration of t and strong static dipolar interactions between, 3C... 

Some of the pulse sequences involve additional refocusing pulses in the l3C channel, the proton channel, or both channels. This discussion and examples presented here are confined to the simplest of interrupted-decoupling sequences, as illustrated in Fig. 4.5.2, since the reader may not have access to the computer code for the more sophisticated versions. 

Lower the "interrupted-decoupling spectrum of the same material. Only i3C signals in the mobile polyether soft segment and the non-protonated i3C resonances in the hard segment survive the 50 ps dipolar dephasing delay. 

An experiment that is useful for assigning nonprotonated carbon resonances in the NMR spectrum is the interrupted-decoupling CP experiment [74,75]. In this experiment, the decoupling of the protons is interrupted (by turning off the proton rf field) for a predetermined time. During the interrupted-decoupling period, 3C... 

Fig. 8.23. An example of the interrupted-decoupling experiment for suppression of the protonated carbon resonances is shown. The labeled peaks correspond to the labeled carbons in the structure. (Reproduced from Ref. [76], 1984 C. Shields.)...

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