Delayed decoupling

Fig. 8 Schematic diagrams for the following pulse sequences (A) single pulse excitation/magic-angle spinning, (B) total suppression of sidebands, and (C) delayed decoupling, or dipolar dephasing.

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.

Rarefaction waves are generated circumferentially at the tube as the detonation leaves then they propagate toward the tube axis, cool the shock-heated gases, and, consequently, increase the reaction induction time. This induced delay decouples the reaction zone from the shock and a deflagration persists. The tube diameter must be large enough so that a core near the tube axis is not quenched and this core can support the development of a spherical detonation wave. 

Figure 6. 31.94 MHz 13C NMR spectra for suberized cell walls from potatoes, before (bottom) and after (top) depolymerization treatment. The experimental parameters were as in Figure 4. Chemical-shift assignments and relative numbers of carbons for the untreated material are found in Table IV. Delayed-decoupling experiments left some (CH2) signal intensity in the spectrum of intact suberin, but the analogous signals were drastically attenuated in the NMR spectrum of the depolymerization residue.

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... 

Nitration of poly-DCH. In contrast to the bromination reaction, the nitration reaction cannot be as easily controlled, and the resultant products are all highly nitrated. The results of some preliminary studies are shown in Fig. 5, where the spectra of a sample of composition poly-DCH(N02)7.6 and the model compound N-ethyl-3,6-dinitrocarbazole are compared. The sites of nitration, unlike those of bromination, can be directly observed in the CP-MAS NMR spectra, although they are somewhat broadened by the dipolar interaction with the quadrupolar nitrogen nucleus as discussed above. No triple bond resonance is detected in the nitrated polymer, while some poorly resolved absorption in the region near 130 ppm in the spectra obtained with delayed decoupling indi-... 

Solid state NMR spectra of isotactic PMEPL prepared by solution casting and melt crystallization are shown in Fig. 1. The peak assignments were confirmed by delayed decoupling spectra. Comparison of the two spectra of Fig. 1 indicates significant differences between the melt ciystallized and the solution cast samples. In the melt crystallized spectrum, both C3 and C4 (defined in Fig. 2) appear as two well separated peaks rather than as single resonances, with the major components of these peak pairs located at 66.1 and 23.4 ppm, respectively. The NMR spectrum of the solution cast film contains only one peak for each of C3 and C4, located at 74.1 and 18.1 ppm, respectively. These chemical shifts correspond to those of the minor components of the two peak pairs... 

Fig. 7. A C-13 relaxation time measurement of solid state wetted cellulose acetate (6% by weight water) using the inversion recovery (IR) method at 50.1 MHz and spinning at 3.2 kHz at the magic angle (54.7 deg) with strong proton decoupling during the aquisition time (136.3 ms), (upper part of the Figure). Tau represents the intervals between the 180 deg (12.2 us) inverting and 90 deg (6.1 us) measuring pulse. 2200 scans were collected and the pulse delay time was 10 s, Cf. Table 3 and Ref.281...

NMR Spectroscopy. All proton-decoupled carbon-13 spectra were obtained on a General Electric GN-500 spectrometer. The vinylldene chloride isobutylene sample was run at 24 degrees centigrade. A 45 degree (3.4us) pulse was used with a Inter-pulse delay of 1.5s (prepulse delay + acquisition time). Over 2400 scans were acquired with 16k complex data points and a sweep width of +/- 5000Hz. Measured spin-lattice relaxation times (Tl) were approximately 4s for the non-protonated carbons, 3s for the methyl groups, and 0.3s for the methylene carbons. 

Composite pulses reduce the error introduced due to the delay between the start of the pulse and when it reaches full power. They have also been used to overcome problems of sample overheating during broadband decoupling and in experiments in which pulses have to be applied for long durations. 

The decoupler is off during the first l/J delay period, so it is during this period that the effect of /-splitting comes into play and the coupling information is provided. This information is contained in the phase and magnitude of the signal, which in turn are dependent on the positions. 

The INEPT experiment can be modified to allow the antiphase magnetization to be precessed for a further time period so that it comes into phase before data acquisition. The pulse sequence for the refocused INEPT experiment (Pegg et al., 1981b) is shown in Fig. 2.13. Another delay, A. is introduced and 180° pulses applied at the center of this delay simultaneously to both the H and the C nuclei. Decoupling during data acquisition allows the carbons to be recorded as singlets. The value of Z), is adjusted to enable the desired type of carbon atoms to be recorded. Thus, with D, set at V4J, the CH carbons are recorded at VsJ, the CH2 carbons are recorded and at VeJ, all protonated carbons are recorded. With D3 at %J, the CH and CH ( carbons appear out of phase from the CH2 carbons. 

The basic INEPT spectrum cannot be recorded with broad-band proton decoupling, since the components of multiplets have antiphase disposition. With an appropriate increase in delay time, the antiphase components of the multiplets appear in phase. In the refocussed INEPT experiment, a suitable refocusing delay is therefore introduced that allows the C spin multiplet components to get back into phase. The pulse sequences and the resulting spectra of podophyllotoxin (Problem 2.21) from the two experiments are given below ... 

Both experiments are based on polarization transfer from sensitive nuclei to insensitive nuclei, and therefore the mjyor portions of their pulse sequences are common. The INEPT experiment, without refocusing and decoupling, however, yields spectra with distorted" multiplets. For instance, the two lines of a doublet appear in antiphase with respect one another. Similarly, the central line of a triplet may be too small to be visible, while the outer two lines of the triplet will be antiphase to one another. Introducing a variable refocusing delay A and broadband decoupling in the INEPT sequence can convert this experiment into a more useful one. 

Fig. 37 Nitrogen-15 spectra of two aminophosphonates (structures as shown). 10-mm NMR tube, concentration 25% in CDC13, proton decoupling, relaxation delay 15 sec, measurement time 12 hours...

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