Pulsed decoupling excitation

Fig. 2. Pulse sequence of the rotary resonance recoupling experiment (R ) where the amplitude of u>2 matches an integer multiple of Wr. Here we have assumed a spin system with additional presence of H spins in the sample, hence the pulse sequence includes (ramped) cross polarization H— S and H decoupling during acquisition. This assumption will be made throughout as this represents the most typical circumstance. However, replacing CP by direct single-pulse Si excitation and omitting the H decoupling, the R pulse sequence and all sequences discussed in the following are also applicable to (Si, S2) spin systems in the absence of H spins.

Proton NMR peaks produced in liquid crystals by soft excitation pulses are considerably narrower than what can be achieved with multi-pulse decoupling, and such signals can be used for very accurate measurement of coefficients of selfdiffusion. With this type of excitation, the radio-frequency power is at least six orders of magnitude smaller than an average power needed for homo- or hetero-nuclear proton decoupling. Therefore, the radio-frequency heating of the sample does not affect the measurements. 

The semiselective pulses in the previously described experiments were employed for either excitation or decoupling, but they were not used for... 

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.

Pig. 1. Pulse sequence for selective reverse INEPT. The time-shared homonuclear decoupling during acquisition is optional, and a variety of simplifications may be made to the sequence depending on the instrument used and on the spin system under investigation, as discussed in the text. A DANTE sequence is shown as the selective 90° carbon-13 pulse, but this may be replaced by a soft pulse or some other form of selective excitation. Phase cycling for this sequence is summarized in table 1. 

Figure 14.8 (a) General scheme of two-dimensional multiple-quantum (MQ) spectroscopy. During both intervals of free precession (MQ coherences during tx and SQ coherences during t2) dipolar decoupling can be achieved by MAS. Possible pulse sequences used for excitation/reconversion of MQ coherences are (b) broadband... 

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...

The standardized pulse program for a proton decoupled 13C spectrum is shown in Figure 4.2a. The sequence is relaxation delay (Rd) (see Section 4.2.3), rf pulse (6), and signal acquisition (t2). The proton channel has the decoupler on to remove the H—13C coupling, while a short, powerful rf pulse (of the order of a few microseconds) excites all the 13C nuclei simultaneously. Since the carrier frequency is slightly off resonance FID (free induction decay), for all the 13C frequencies, each 13C nucleus shows a FID, which is an exponentially decaying sine wave. 

The most recent research efforts in FDCD have been concerned with developing analytical methods that are sensitive to fluorescent chiral systems. No doubt this will continue to be a driving force for experimental and conceptual advances in this field. One of the most interesting aspects of the technical improvements described in reference [21] for the measurement of time-resolved CPL is the fact that, by employing randomly spaced (in time) excitation pulses, the time scale for measurement has been decoupled from the 50 kHz PEM modulation cycle. In principle, this same approach could be used in FDCD measurements. Thus, it should be possible, for example, to measure time-resolved FDCD from fairly long-lived chromophores such as lanthanide (III) ions. 

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