Spin-lock effect

A so-called spin-lock effect manifests itself in a dependence of the effective transverse relaxation time Tie on the echo time tn of the multi-echo sequence [Gut3]. 

As was shown in articles, the preparatory pulse effect is "forgotten" by the spin system after times of abouf several T2e and, consequently, the spin-locking effect vanishes at the same time. If in Equation (43) we set pg = 0, the result will be that "forgetting" the preparatory pulse leads to the disappearance of fhe differences between MW-4 and SORC. 

Sequence (71) with the preparatory pulse phase 4> = does not create a spin-locking effect (Figure 11). Therefore using a combination of two sequences... 

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 ... 

The use of spin-lock pulses for water suppression is illustrated with the NOESY and ROESY pulse sequences (fig. 5). Using the Cartesian product operator description [9], the effect of the NOESY pulse sequence of fig. 5(A) is readily illustrated ... 

More serious are the coherence transfer cross peaks in ROESY spectra because the coherence peaks are in phase with the genuine cross-relaxation peaks and thus may modulate intensity of the genuine peaks. To emphasize the effect of coherence transfer peaks (now TOCSY peaks) we do the ROESY experiment with Tm = 300 ms and with a spin-lock field of 5 kHz (fig. 4(C)). Besides positive diagonal peaks (thick contours), several pairs... 

Fig. 4. Pulse sequences for determining spin-spin relaxation time constants. Thin bars represent 7t/2 pulses and thick bars represent tt pulses, (a) the CPMG sequence, (b) the spin-lock sequence used for determining T p and (c) a two-dimensional proton-detected INEPT-enhanced CPMG. T is the waiting period between individual scans. The pulse train during the T period is used for suppression of cross-correlation effects, and the delay S is set to < (1/2)J. The delay A in (c) is set to (1/4)Jih and A is set to (1/4)Jih to maximize the intensity of IH heteronuclei and to (1 /8) Jm to maximize the intensity of IH2 spins. The phase cycling in (c) is as follows i = y,—y 2 = 2 x),2 —x), i = 8(x), 8(—x) Acq = x, 2(—x), x, —x, 2(x), —x, —x, 2(x), —x,x, 2(—x), x. The onedimensional version the proton-detected experiment can be obtained by omitting the ti delay.

These questions were resolved with the use of the same relatively simple epoxy system. All C-13 nuclei in contact with the proton bath were counted when moderate spinning rates were used and in spin-lock cross polarization in rf fields not close to any Tle minimum. The molecular motion determines the relaxation rate, under the Hartmann-Hahn condition when T, = T2. The spin-spin effects determine relaxation when Tle does not equal T2 under the same conditions 62). The spin-spin fluctuations are in competition with the spin-lattice fluctuations in producing an effective relaxation time. To discriminate against the spin-spin fluctuations large rf fields are mandatory. It was pointed out that, with great care, C-13 NMR spectra can reflect molecular motion. 

This H experiment (Fig. 3.10) serves a similar purpose as the NOE experiment. In the NOE experiment, relaxation processes occur in the presence of the strong static magnetic field and a weak selective rf field. The ROE experiment is based upon cross relaxation processe.s (T.p) observed between spins, that occur in the presence of a transverse, weak spin-locking rf field (either a continuous CW rf field or a series of weak rf pulses). According to theory, and in contrast to NOEs, ROEs are positive irrespective of the size or mobility of molecules and no unwanted zero-passing of the effect exists. However the effects at the small and large molecular size limits are both smaller compared to the corresponding NOE values of 0.5 and -1 respectively. The ROE experiment is ideal for intermediate sized molecules where NOEs may be close or equal to zero. 

The ROE dependence on the spin lock time has the same profile as that of transient NOE, with the difference that the limiting values are 0.385 and 0.675 at the condition that o>itc < 1 (see Fig. 7.10). It appears that the ROE is less convenient than the transient and steady state NOEs in the sense that the expected effect is smaller when all other conditions are the same. Another disadvantage in paramagnetic molecules is that it is difficult to spin lock all the signals in a... 

Carbon resonances arising from both nonprotonated and proto-nated aromatic carbons may appear at the same frequency under proton decoupling. Yet these two resonances could possess very different relaxation behavior and in a solid could evolve very differently due to local proton dipolar fields which attenuate with the carbon-proton distances as 1/rcH When the spin locking pulse for proton nuclei is turned off, carbons with directly bound protons such as methines and methylenes rapidly dephase in the local proton fields and their spectral response is rapidly diminished. The rapid internal motion of CH3 groups greatly decreases the effectiveness of methyl protons. Nonprotonated carbons are only dephased by remote and therefore... 

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