Pulsed Proton Broadband Decoupling

Looking at the series of subspectra obtained with both sequences, the perfect suppression of central signals is noticeable. This must be attributed on the one hand to the highly effective combination of proton broadband decoupling, the purge pulses and phase cycling and on the other hand to... 

Fig. 2.48. INADEQUATE pulse sequence (a I) and pulse-driven motion (a-f) of carbon-13 magnetizations S0 for UC —12C singlet and S2 for 13C — 13C doublet signals in the rotating frame. Proton broadband decoupling is applied throughout the experiment.

In pulsed or gated decoupling of protons (broadband decoupling only between FIDs), coupled C NMR spectra are obtained in which the CH multiplets are enhanced by the nuclear Overhauser effect (NOE, see Section 1.9). This method is used when CH coupling constants are required for structure analysis because it enhances the multiplets of carbon nuclei attached to protons the signals of quaternary carbons two bonds apart from a proton are also significantly enhanced. Figure 1.10 demonstrates this for the carbon nuclei in the 4,6-positions of 2,4,6-trichloropyrimidine. 

Figure 19-33a is a C 2D NMR spectrum for a COSY experiment with 1,3-butanediol. Figure 19-3.3a is the ordinary one-dimensional spectrum for the compound. The two-dimensional spectrum is obtained as follows With the proton broadband decoupler turned off, a 90 pulse is applied to the sample. After a time the decoupler is turned on, another pulse is applied, and the resulting FID is digitized and transformed. 

The pulse sequence which is used to record CH COSY Involves the H- C polarisation transfer which is the basis of the DEPT sequence and which Increases the sensitivity by a factor of up to four. Consequently, a CH COSY experiment does not require any more sample than a H broadband decoupled C NMR spectrum. The result is a two-dimensional CH correlation, in which the C shift is mapped on to the abscissa and the H shift is mapped on to the ordinate (or vice versa). The C and //shifts of the //and C nuclei which are bonded to one another are read as coordinates of the cross signal as shown in the CH COSY stacked plot (Fig. 2.14b) and the associated contour plots of the a-plnene (Fig. 2.14a and c). To evaluate them, one need only read off the coordinates of the correlation signals. In Fig. 2.14c, for example, the protons with shifts Sh= 1.16 (proton A) and 2.34 (proton B of an AB system) are bonded to the C atom at c = 31.5. Formula 1 shows all of the C//connectivities (C//bonds) of a-pinene which can be read from Fig. 2.14. 

The T, Inversion-Recovery experiment is not restricted to C nuclei, but may also be applied to other nuclei, e.g, protons. In this case, the pulse sequence for the observe channel is the same, but no broadband decoupling is used. 

H and 19F NMR spectra are recorded with a normal one-pulse sequence or, alternatively, the XH spectra are recorded with a sequence that allows simultaneous solvent suppression with presaturation (31) or a sequence that includes some other method of suppression 13C 1H and 1P 1H spectra are recorded with proton broadband (composite pulse) decoupling (32), and 31P spectra with gated proton decoupling (33). 

As we have seen, the DQC and ZQC evolve at frequencies f s flT. The 180° proton (I,S) pulse, which is a standard part of the HMQC sequence, refocuses proton chemical shifts that evolve during t2 and interchanges ZQC and DQC. The final 90° T (15N) pulse converts SxTy to SXTZ, and this antiphase 5 magnetization evolves during the final 1/2/ period to in-phase S magnetization, which is detected while T is broadband decoupled. 

Fluorine-19 NMR data were acquired at a frequency of 188.22 MHz with a Varian XL-200 spectrometer. Typically, 100 transients were accumulated from a 5% polymer solution by volume in dimethylformamide-d7 placed in a 5 mm sample tube at 120 C with internal hexafluorobenzene as a reference ( = 163 ppm). A sweep width of 8000 Hz was used with 8 K computer locations (acquisition time 0.5s) and a 5.0 s delay between 90 pulses (9.0 s duration). Proton heteronuclear coupling was removed by broad-band irradiation centered at 200 MHz. A modified Bruker WH-90 spectrometer allowed carbon-13 NMR spectra to be obtained with simultaneous proton and fluorine-19 broadband decoupling (13). 

As described in Section 4.4 for pulsed Fourier transform spectrometry of protons, a short, powerful, rf pulse (on the order of a few microseconds) excites all of the 13C nuclei simultaneously. At the same time, the broadband decoupler is turned on in order to remove the 13C— H coupling. Since the pulse frequencies are slightly off resonance for all of the nuclei, each nucleus shows a free induction decay (FID), which is an exponentially decaying sine wave with a frequency equal to the difference between the applied frequency and the resonance frequency for that nucleus. Figure 5.2a shows the result for a single-carbon compound. 

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