Solid-echo sequences

The slice selection procedure can be combined with a number of pulse sequences to spatially resolve NMR parameters or to contrast the profiles with a variety of filters. The most commonly used acquisition schemes implemented to sample echo train decays are the CPMG [(jt/2)0—(Jt)90] or a multi-solid echo sequence [(jt/ 2)0-(jt/2)9o]. In these instances, the complete echo train can be fitted to determine... 

Because the solid echo can reduce the deadtime problem, it can be used in conjunction with other pulse sequences to detect signals which are otherwise difficult to see. In particular, the solid echo sequence can be used in place of the monitoring pulse in any of the T sequences mentioned in Section III.D.2. 

Hahn echo sequences work regardless of the relative phases of the two pulses so that the solid echo sequence (7i/2)Q-x-(7i/2)gQ can induce Hahn echoes in an isotropic non-viscous liquid as well as the echoes from two identical n/2 pulses. Therefore, two different contributions to an echo can exist for the solid echo in a sample like a liquid crystal which has both a liquid and solid character (Cohen-Addad, 1974 Cohen-Addad, and Vogin, 1974). 

Now, in isotropic and non-viscous liquids, the Hahn echoes from a n/2-i-n sequence will be about twice as large as that from a n/2-x-n/2 sequence if there are no instrumental problems such as H inhomogeneity (see H.D.5.). On the other hand, solid echoes, in principle, can recover the full height of the FID. Therefore, the maximum echo height from a solid echo sequence in a sample with both liquid and solid character is sensitive to the ratio of the Hahn and the solid echo components. In particular, if it is close to the full... 

Other pulse sequences leading to echo formation in solids containing two different spin systems are the two with a pair of ti/2 pulses including the solid echo sequence for one kind of spin, i.e., (7i/2)Q-x-(7i/2)Q-x-echo and (ti/2)q-t- (n/2)gQ-T-echo (Mansfield, 1965). The first of these is sensitive to the dephasing due only to dissimilar nuclei. Thus, by a judicious choice of pulse sequences, is is possible to distinguish the separate contributions to the dephasing in a two-spin system. 

The maximum S/N gain expected from cross-polarization relative to a solid-echo sequence is given by the ratio of the nuclear gyromagnetic ratios and the heat capacities of the two spin reservoirs [33,34]. Equation (3) describes the change of magnetization from a reference state, Mgo (the equilibrium magnetization obtainable in the spin-echo experiment), to a final equilibrium state,... 

One technique that is often used to alleviate the effects of receiver recovery is the solid echo sequence (Figure 7.2(a)) [18], where a second 90° pulse (phase shifted by 90° with respect to the first) is applied to the system after an interval that is slightly longer than the dead time. This generates an echo... 

Fig. 6. The generalized Jeener-Broekaert three pulse sequence. Note that FT of the solid echo and the alignment echo starts at times delayed by the pulse separation r, after the second and third pulse, respectively...

Figure 4. Dependence of the integrated intensity on the length between pulses, T, in the solid spin echo sequence. C6De/(Cs,Na)X at 2.4 molecules/supercage and 283 K. SW = 500 kHz.

Users of any NMR instrument are well aware of the extensive employment of what is known as pulse sequences. The roots of the term go back to the early days of pulsed NMR when multiple, precisely spaced RF excitation pulses had been invented (17,98-110) and employed to overcome instrumental imperfections such as magnetic field inhomogeneity (Hahn echo) or receiver dead time (solid echo), monitor relaxation phenomena (saturationrrecovery, inversion recovery, CPMG), excite and/or isolate specific components of NMR signals (stimulated echo, quadrupole echo), etc. Later on, employment of pulse sequences of increasing complexity, combined with the so-called phase-cycling technique, has revolutionized FT-NMR spectroscopy, a field where hundreds of useful excitation and detection sequences (111,112) are at present routinely used to acquire qualitatively distinct ID, 2D, and 3D NMR... 

Figure 12 Offset dependence of the intensity of the CT resonance in MAS NMR spectra of Na2MgEDTA-4H20, recorded with a single HS pulse placed before the spin-echo sequence. The experimental data (square points) were recorded using a 7mm probe (with 7kHz MAS rate) the solid red line is just a guide for the eyes. Also shown (blue line) is the simulated first-order quadrupole static powder pattern for the single Mg site in this compound.

The analysis of the data of PS I gave quite accurate information on the distance of the spin centres (25.4 0.3 A)301 that compared well with the crystal structure data.68 A problem is the extended it-spin density distribution in the donor and acceptor. For a solid comparison a centre of gravity for the spin must be calculated from experimental or theoretical spin density distributions of the two radicals. Similar data with almost unaltered distances were obtained for PS I with other quinones substituted into the Ai site.147-302This work has been extended to other electron acceptors,303 which show a larger heterogeneity in distances. It has been shown that the lifetime of the RP can also be measured and can even be controlled in the experiments by an additional mw pulse prior to the 2-pulse echo sequence.302 This pulse transfers population to triplet levels which cannot directly recombine to the singlet ground state. This has earlier been shown for the bRC.304,305 The OOP-ESEEM technique has also been applied to various mutants of PS I to characterize them by the measured distances between fixed donor and variable acceptors.254 256-263-264... 

Since the spin-spin relaxation of the liquid phase naphthalene protons is much longer than that of the solid naphthalene protons, by once more using the spin-echo sequence with x =. 007 s we are able to monitor the NMR signal from the liquid phase only. 

Figure 10.15 The decay of the transverse magnetisation (points) for ethylene-octene copolymer at different temperatures [136]. The decay was measured using the solid-echo pulse sequence. The solid lines represent the result of a least-squares adjustment of the decay using a linear combination of Weibull and exponential functions. The dotted lines represent the relaxation component with a long decay time. In the experiments the sample was heated from room temperature to 343 K (70 °C)...

Fig. 24. Top Magic-echo sequence, TREV-16TS, for solid-state imaging with (a) ideally pulsed (b) nonideally pulsed and (c) sinusoidal field gradients. On-resonance RF irradiations along x and — x are sandwiched by 90° pulses (magic sandwich). The hatched magic sandwich consists of four magic sandwiches. Bottom Projections of the one-dimensional test sample (admantane powder) obtained with the TREV-16TS sequence (r = 30 ps) and the field gradients shown in Fig. 18. The RF field was set at 100 kHz. (Adapted from Matsui et al.296 with permission.)...

Figure 7. Typical pulse sequences applied for obtaining an 2H NMR solid echo (top) and a stimulated echo (bottom).

Exchange in solids can be studied by NOESY, except that the preparation period consists of a cross polarization sequence, rather than just a 90° pulse. Similar studies can also be carried out with 2H NMR by combining the 2D pulse sequence with the solid echo described in Chapter 7. 

As a second example, we look at echoes. We saw in Chapter 9 that a 180° pulse refocuses not only chemical shifts and the effects of magnetic field inhomogeneity but also spin coupling provided that the pulse does not also disturb the spin state of the coupled nucleus (see Fig. 9.2) However, in a homonuclear spin system a nonselective pulse does effect spin states. We found in Chapter 7 that dipolar interactions have the same mathematical from as indirect spin coupling, and it is known that a 180° pulse does not produce an echo in a solid because spin states are disturbed. However, it is possible to obtain a solid echo or dipolar echo by applying the pulse sequence 90, T, 90r It is very difficult to rationalize an echo from... 

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