Transfer time, spin-lock

Under MAS the quadrupole splitting becomes time dependent, Qg = Qg (f) (see Sect. 2.3.4). This influences both the spin-locking behavior [223] and the polarization transfer [224], with the latter being further affected by the periodic modulation of the IS dipolar interaction. The effect of MAS on spin-locking of the S magnetization depends on the magnitude of the so-called adiabaticity parameter ... 

Fig. 10.23. Cross-polarization pulse sequence. The high abundance nuclei, such as protons, are first irradiated with a standard 90° pulse to create the initial magnetization. A special pair of spin-locking pulses is applied during a period called the contact time in order to transfer the magnetization from the protons to the low abundance nuclei, such as carbons. Protons are then decoupled from carbons during the acquisition of the carbon signal. In the case of protons and carbons, cross-polarization can enhance the observed carbon signal by as much as four-fold.

Fig. 8. ID ROESY-TOCSY. (a) H spectrum of the oligosaccharide 3 (5 mg/0.5 ml D2O). (b) ID ROESY spectrum of 3 acquired using the pulse sequence of fig. 7(a) with selective excitation of the H-lb proton. Duration of the 270° Gaussian pulse and the spin-lock pulse ( yBi/ K = 2.8 kHz) was 49.2 ms and 0.5 s, respectively. The spin-lock pulse was applied 333.3 Hz downfield from the H-lb resonance. The time used for the frequency change was 3 ms. (c) ID ROESY-TOCSY spectrum acquired using the pulse sequence of fig. 7(c) and the selective ROESY transfer from H-lb followed by a selective TOCSY transfer from H-4c. Parameters for the ROESY part were the same as in (b). A 49.2 ms Gaussian pulse was used at the beginning of the 29.07 ms TOCSY spin lock. 256 scans were accumulated. A partial structure of 3 is given in the inset. Solid and dotted lines represent TOCSY and ROESY...

The initial building-up of the carbon magnetization via polarization transfer is characterized with the spin-lock cross-polarization time Tcir It is sensitive to the static interactions, i.e. transfer of the polarization via static dipolar interactions (proportional to e J). Hence, the shortest TCH will have carbons a) in more rigid systems and b) with more directly attached protons, particularly the CH2 (or... 

The 2D ROE or ROESY experiment is an experiment to measure cross-relaxation in the rotating frame (Fig. 8.2B). After an initial 90° pulse and the variable evolution period t, a low power or soft spin-lock sequence (SL) is applied for a time during which magnetization transfer in the rotating frame occurs due to cross relaxation. Since scalar connectivities can also develop during spin lock, as... 

MLEV17 sequence being one of the most used sequences [23]) in such a way as to continuously refocus the chemical shift evolution of the various signals in the xy plane. Analogously to ROESY experiments, the magnetization during the spin-lock (mixing) time disappears with T p (i.e. essentially Tj, see Section 3.4). It follows that coherence transfer in the xy plane, which is built up with a sin(7T J/jt) function, also decreases with time constant p p — p[p + p p)/2 ... 

Even with the line-narrowing techniques described earlier, NMR experiments on solids with dilute spin- /2 nuclei are still relatively unattractive on two principal counts. One is the lack of sensitivity due to their low net polarisation and the other is the relatively long spin-lattice relaxation time that is often encountered. In solids where both abundant (I) and dilute (S) nuclei coexist, polarisation transfer techniques can usually be used to overcome both these problems. There are many schemes to effect such a transfer but the most common technique is to create and then spin-lock transverse I-magnetisation. This experiment is best understood using ideas from spin thermodynamics. The magnetisation is given by Curie s Law (Eq. 2.21) and the temperature in... 

In this equation Tis determines the rate of polarisation transfer and hence the build up of signal, while Tip is the relaxation time of the spin-locked I-magnetisation in the rotating frame determining the time scale of the decay of the reservoir of magnetisation. The rate of transfer of the magnetisation is found to depend on the second moments (M2) of the I-S and I-I spin systems as... 

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