Pulsed decoupling

Broadband proton-decoupled pulse Fourier transform C n.m.r. were recorded in deuterochloroform at 20 MHz using a Varian CFT-20 spectrometer. 

Fig. 9 Examples of simplifying solid state NMR spectra by the TOSS and delayed decoupling pulse sequences. Shown is a comparison of the 31P CP/MAS NMR spectrum of fosinopril sodium utilizing the standard pulse sequence (A) and the TOSS routine (B). Also shown is the full 13C CP/MAS NMR spectrum of fosinopril sodium (C) and the nonprotonated carbon spectrum (D) obtained from the delayed decoupling pulse sequence utilizing a 80 /us delay time. Signals due to the methyl carbon resonances (0-30 ppm) are not completely eliminated due to the rapid methyl group rotation, which reduces the carbon-proton dipolar couplings.

As with any other homonuclear decoupling, the transverse magnetization of the 13C , for example in the evolution time, is perturbed by the decoupling pulse, resulting in an additional precession virtually around the z axis. Since the adiabatic decoupling is applied during the entire evolution time of the 13C , a non-linear frequency shift rather than a phase shift appears in the spectrum. This is termed the Bloch-Siegert (frequency) shift in the NMR literature in honour of their discovery of the phenomenon. 

Fig. 15. Adiabatic decoupling of 13CO from 13C with a compensating pulse applied on the other side of the peaks. The compensating and decoupling pulses have the same shape but opposite frequency sweep. Due to the Bloch-Siegert effects, both the left and the right peaks are pushed towards the center while the centre peak is balanced and remains in its position. Reprinted from Ref. 47 with permission from Elsevier.

The spectrum at the bottom of Fig. 16 is obtained with the double adiabatic decoupling pulse, one located at —23.2 kHz and the other at 23.2 kHz. The BSFS is compensated and sidebands are eliminated by the compensating pulse. In addition, the amplitude of the peak is higher than that in the middle, showing a better decoupling effect. Similar results were obtained for 13C off-resonance <5 ranging from —3 to 3 kHz, where < /A/<0.13 can be treated as close to on-resonance. 

Selective population or polarization transfer (abbreviation SPT) was first achieved for chloroform [51] by brief irradiation of one of both 13C satellites in the proton NMR spectrum before recording the 13C—1H doublet as FID signal. In this experiment, the decoupling pulse inverts the population of the proton precession states connected by the irradiated satellite transition (Fig. 2.43(b, c)). The two SPT experiments shown for the 13C —H doublet of chloroform in Fig. 2.43 (b, c) give an impression of the signal enhancement achievable by population transfer in comparison to a normal 13C NMR spectrum... 

Wilson, M. A., and Goh, K. M. (1977). Proton-decoupled pulse Fourier-transform 13C nuclear magnetic resonance of soil organic matter. J. Soil Sci. 28, 645-652. 

Subsequently, Uhrinova et al.29 reconsidered the problem using both proton-and carbon-detected experiments. For example, couplings of anomeric carbons were measured from the 13C satellites in proton NMR spectra. The critical factor in these methods is the suppression of signals from protons bound to, 2C atoms. In the pulse-sequence proposed, these protons were selectively inverted by a BIRD (Bilinear Rotation Decoupling) pulse,30 and the spin-echo method introduced by Bendall et al.31 was used. 

Figure 14.11 Basic 2D pulse sequence used for the measurement of residual dipolar local fields. After the excitation pulse the spins are allowed to evolve for some time tj (indirect dimension) under influence of the relevant spin interactions before crosspolarisation takes place. The direct detection during time t2 then takes place on the 13C side typically under proton dipolar decoupling (DD). The basic scheme can be extended by various spin manipulation techniques (not shown) during time tj. For instance, the heteronuclear dipolar contribution can be removed by a decoupling pulse...

FIGURE 4.5 Gated proton decoupling pulse sequence. Rd is relaxation delay, 6 is a variable pulse angle, t2 is the acquisition time. 

There are more advanced experiments such as DEPT (Chapter 7) that observe 13 C and use the decoupler to supply high power, short duration ( hard ) pulses at the XH frequency. This requires full power from the decoupler, but the parameters dpwr andpll7 are avoided for these pulses. Setting decoupler power to the maximum might lead to disastrous mistakes because the decoupler can only deliver full power for short ( 10 pis) periods of time without burning up the decoupler, the probe, and the sample. Instead, the parameters pp (Varian) and p3 (Bruker) are used for the 90° pulse width for decoupler hard pulses and pplvl (Varian) and pl2 (Bruker) indicate the power level for short-duration high-power decoupler pulses. 

Usually a homonuclear decoupling pulse sequence does not simultaneously decouple heteronuclear dipolar interactions, albeit in many cases the heteronuclear dipolar couplings need to be retained. With a significantly modified magic-sandwich sequence, Schmidt-Rohr proposed a pulse sequence that can eliminate both, at least up to first order.63 This sequence is particularly useful for enriched 13C pairs and can be readily combined with 2D experiments. It would be interesting to investigate whether such a sequence or similar ones can work for other nuclei particularly large-7 species. 

The new decoupling pulse sequences CNvn as shown in Fig. 4 were found to be competitive with TPPM. Furthermore, as can be seen from Fig. 5, they... 

Two situations now confront the inorganic chemist—interest in H X direct and H,X indirect spectroscopy in these cases the X decoupler pulses will be on the first decoupler channel or interest in samples where neither nuclide is proton In this case, the X decoupler pulses will be on the second decoupler channel, variously denoted, second decoupler channel , Y channel , G , etc. Consult the manufacturer s apphcations department for advice if you are not sure. 

H[X Direct and H,XIndirect Spectroscopy. This is no different from setting up the decoupler pulses for H C —you will need a sample in which H couples to X. 

Figure 3 N 90° decoupler pulse measurement via using Bruker pulse program DECP90F3, sample [Pt(C5H5 N)(PPli3)Cl2] in dg-THF. (a) decoupler off-resonance—antiphase doublet does not invert, gives a false result, (b) decoupler on-resonance, correct...

Figure 19 H, N N HSQC spectra of [(N3(P(Bu )2)3] in CDCI3 at 295 K recorded without (a) and with (b)CHIRP60 adiabatic P decoupling pulse. Bruker pulse program hsqcetfigpsi2...

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