Spin-locks

Comment Peak assignments determined with 2D Spin-Lock Relay experiment. 

The pulse sequence for the ID ROESY experiment using purged half-Gaussian pulses is shown in Fig. 7.7. The purging is required to remove the dispersive components, since these are not completely eliminated by the weak spin-lock field employed in the ID ROESY experiment. 

Spin-locking If a continuous field is applied along the y -axis immediately... 

In the delayed-contact experiment a variable delay, during which proton spin-locking was maintained, was inserted prior to a fixed contact time of 0.5 ms. 

Parhcular care has to be taken when implementing ROESY experiments. The spin-lock, which holds the spins along a defined axis perpendicular to the stahc magnetic field, can be realized in many different ways and is shU an achve field of research [18, 20]. In most spin-lock sequences the conditions for undesired TOCSY transfer are parhally fulfilled and especially cross-peaks close to the diagonal or anhdiagonal might not be accurately interpretable. Since in most cases the effechveness of the spin-lock also depends on the chemical shift offset, an offset-dependent correction has to be applied to the measured cross-peak intensities [20]. 

Figure 6 X-half filters used for filtering or selecting 13C and 15N-attached protons. Thick and thin closed rectangles are 180° and 90° pulses, respectively, open rectangles are spin lock pulses. (A) A simple X-half filter (2). The delay t is equal to 0/(2[1JXH]) where 1JXH is the one-bond coupling between proton and either 13C (120 to 140 Hz) or 15N (95 Hz). The second 90° pulse is the editing...

In addition, the technique of cross polarization introduced and developed by Pines, Gibby and Waugh (9) is used to increase the signal-to-noise ratio of the spectrum. The proton magnetization is spin-locked along the y axis with a spin-locking field % and the carbons subjected to an RF pulse chosen such that the two fields fulfill the Hartmann-Hahn condition (10), equation [3] (Figure 2). 

Recently a new type of proton assisted recoupling experiments has been developed for coherence transfer where rf irradiation is taking place on all involved rf channels. This embraces the homonuclear proton assisted recoupling (PAR) [45, 140, 141] and the later resonant second-order transfer (RESORT) [142] experiments, as well as the heteronuclear proton assisted insensitive nuclei (PAIN) cross polarization [44] experiments. In PAR and PAIN, spin-lock CW irradiation is applied on both passive ( H) and active spins (13C, 15N) without matching rotary resonance conditions. In RESORT a phase alternation irradiation scheme for the passive spins is used. 

In contrast to the spin-1/2 —> spin-1/2 polarization case, the transfer of polarization between spin-1/2 and quadrupolar nuclei presents a considerable challenge due to the very complex spin dynamics involved in the CP process and the spinlocking of quadrupolar nuclei under MAS. The overall sensitivity is rarely enhanced with respect to the direct polarization method, even if T[ is much shorter than Tf. The usefulness of the CP experiment relies mainly on its ability to probe the interactions between the neighboring nuclei in ID or 2D schemes under MAS or isotropic resolution. The theoretical and practical aspects of spin-locking and CP transfer between spin-1/2 and the SQ or MQ coherences of the quadrupolar nuclei (SQ-CP or MQ-CP) have been thoroughly studied and reviewed [20, 221-227]. Some of the challenges involved in such experiments are summarized below. 

The spin-locking and CP behavior of the most commonly used SQ coherence (CT) in quadrupolar nuclei under static and MAS conditions has been described in detail by Vega using the fictitious spin-1/2 approximation [223]. In a static sample, the Hartmann-Hahn matching condition requires that co = nut where co ut is one of the nutation frequencies associated with the SQ coherence of the quadrupolar S spin (see Sect. 2.3.4). In the simple case of on-resonance SQ-CP this translates to [224]... 

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 CP can also be used for polarization transfer to MQ coherences of half-integer quadrupolar nuclei [222, 223, 228-231]. This type of transfer is mainly used in the context of MQMAS [228,229,231], although the spin-locking of MQ coherences is also featured in experiments involving homonuclear dipolar recoupling experiments [232]. 

J splittings cannot be directly resolved. In addition to the obvious advantage of providing a map of chemical bonds between the spins, /-based transfers do not require spin-locking and are not disturbed by molecular motions. The major drawback of polarization transfer through J coupling is that the delays involved in the pulse sequences, such as insensitive nuclei enhanced by polarization transfer (INEPT) [233] or heteronuclear multiple-quantum coherence (HMQC)... 

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