Spin-lock field

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. 

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). 

One can further increase the amount of transferred polarization if one carries out the cross polarization in an adiabatic fashion. In this experiment, the amplitude of one of the spin-lock fields is usually varied in a tangential shape [33-35]. In addition to the compensation of instabilities in the amplitude and rf field inhomogeneities, one can also obtain a gain in signal by a up to a factor of two. The concept of adiabatic polarization transfer will be discussed in more detail in Sect. 11.3.1. 

Jonas et al. measured the proton rotating frame spin-lattice relaxation time (Tip) at pressures from 1 bar to 5000 bar and at temperatures of 50 to 70 °C for DPPC and at 5 to 35 °C for POPC. If intermolecular dipolar interactions modulated by translational motion contribute significantly to the proton relaxation, the rotating frame spin-lattice relaxation rate (1/Tip) is a function of the square root of the spin-locking field angular frequency... 

TOCSY peaks might appear in the GROESY spectrum as if truly due to cross-relaxation. This drawback can be corrected by means of a very careful calibration of the 180° pulses used for the T-ROESY spin-locking field. 

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... 

The easiest way to reduce the amplitude of TOCSY cross peaks in the ROESY spectra is to record a spectrum with minimal spin-lock power [23]. The other possibility is to modulate the frequency of the spin-lock field [25]. However, the most convenient way is to apply a series of 180° pulses instead of a single continuous-wave pulse during the mixing time, as is done in the T-ROESY experiment. Figure 4(D) shows the T-ROESY spectrum of cyclo(Pro-Gly) recorded with = 300 ms. Although the... 

Relaxation in the rotating frame is determined by the strength of the spin-locking field Br Therefore, Tt times are sensitive to the intermediate frequencies co1 = yBt, in the kHz region 9) ... 

The pulse sequence for ICP experiments appears simple a 90° proton pulse is followed immediately by a spin lock radio-frequency (rf) field of strength B that is phase shifted by 90° relative to the first pulse. By a spin-lock field is meant a strong rf field B that is on resonance with the given nucleus it keeps magnetization in a spin-locked orientation parallel to the B direction where the decay of magnetization is governed by T p. At present the strong continuous B field is replaced by multipulse sequences that are well known from other spin-lock experiments such as TOCSY, ROESY etc. Simultaneously,... 

In addition to the difficulty of finding the optimal r, the JCP experiment also suffers from extreme sensitivity to the HaHa match. Moreover, the original experiment required both rf fields (Si and H) to be highly homogenous, preferably created by the same transmitter coil, to keep the same ratio of the two fields within the whole active volume of the sample147. To reduce the sensitivity of the enhancement to the HaHa condition and cross-relaxation time, a modified, refocused JCP experiment (RJCP) was suggested151. In this experiment the spin-lock field on the silicon resonance is interrupted at r = j for a duration corresponding to a conventional 90° pulse, and later (r = /) the spin-lock field on protons is similarly interrupted both fields then remain on until r = 2//. 

Early observations of calcium in the solid state used either double resonance178 in CaF2 or adiabatic demagnetization at low temperature.179 H-43Ca CP was one of the first experiments that reported more conventional NMR spectra.180 CP used a 46 kHz spin-locking field on the protons that was matched to the central transition of the calcium. Optimum CP was achieved with a contact time of... 

A selective TOCSY experiment starts with putting the net magnetization of just one resonance in the x -y plane and locking it with the TOCSY mixing spin lock. After an appropriate mixing time, the spin lock field is turned off and we simply start acquiring the FID. These steps can be summarized as follows ... 

Ti reports on fast dynamics on a timescale of ps-ns, whereas T2 relaxation depends on both fast and slower dynamics (ps-ns and xs-ms). The experimentally measured T2 relaxation times include an exchange contribution that can be measured by a Carr-Purcell-Meiboom-Gill (CPMG) pulse train (25, 26) or an effective spin-lock field (27-29). The combination of T2 and Tip measurements allows determination of the contribution of chemical exchange to the relaxation time. Eurthermore, relaxation dispersion experiments have been developed to measure slow time-scale xs-ms dynamic processes (30-35). 

For many applications, the basis sequence can be iteratively constructed from simplw tarting sequences (Tyko, 1990). MLEV-4-type super cycles RRRR or RRRR (Levitt et al., 1983) are examples of simple iterative schemes for the construction of basis sequences with vanishing effective fields from a starting sequence R, which is a (approximate) composite inversion pulse R. Here, the composite pulse R is identical to R, except that the phases of all square pulses are shifted by 180°. The MLEV-16 super cycle RRRR RRRR RRRR RRRR (Levitt et al., 1983) suppresses effective fields even better. MLEV-4- and MLEV-16-type supercycles are often used in the construction of broadband Hartmann-Hahn mixing sequences. In these sequences, an effective spin-lock field can be introduced by adding an uncompensated additional pulse after each complete supercycle (see Section X). 

As stated in the introduction of this section, we use Hartmann-Hahn experiment as the generic term for transfer experiments that are based on the Hartmann-Hahn principle, that is, on matched effective fields. Because two vanishing effective fields are also matched, Hartmann-Hahn sequences need not have finite effective fields. Examples of Hartmann-Hahn sequences without effective spin-lock fields are MLEV-16 (Levitt et al, 1982), WALTZ-16 (Shaka et al., 1983b) and DIPSI-2 (Shaka et al., 1988). Note that the term Hartmann-Hahn sequence has also sometimes been used in the literature in a more restricted sense for experiments with matched but nonvanishing effective spin-lock fields (see, for example, Chandrakumar and Subramanian, 1985, and Griesinger and Ernst, 1988). 

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