Spin-lock experiments

Fig. 5. Principle of a spin-lock experiment leading to the determination of the relaxation time in the rotating frame (Tip). (SL)y stands for the spin-lock period which corresponds to the application of a rf field along the y axis of the rotating frame.

The transverse cross-relaxation rate does not exist due to the difference in I and S resonance frequencies. It is, however, retrieved (see Section I.D) in a spin-lock experiment (where this difference disappears) ... 

Fig. 1.25. In a spin-locking experiment the magnetization is initially brought along the x axis of the rotating frame (left), and then locked along that axis (right), by proper reorientation of the r.f. field B,.

Complementary NMR measurements, such as rises of carbon polarisation in a spin-lock experiment and determination of 13C spin-lattice relaxation times in the rotating frame, Tip(13C), support these conclusions about the correlation times of the side-ring CH and CH2 motions in the various poly(cycloalkyl methacrylates). 

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,... 

Bax and co-workers demonstrated that a homonuclear Hartmann-Hahn transfer of net magnetization can be obtained by the application of a spin-lock field, using CW irradiation (Bax and Davis, 1985a Davis and Bax, 1985) or by the DB-1 sequence that consists of a series of phase-alternated spin-lock pulses (Davis and Bax, 1985). The homonuclear Hartmann-Hahn effect caused by CW irradiation was discovered when artifacts in ROESY experiments were analyzed (Bax and Davis, 1985a). CW irradiation can be regarded as a homonuclear analog of spin-lock experiments for heteronuclear cross-polarization (Hartmann and Hahn,... 

Despite the inhomogeneous B field of the siuface coil, spin-lock experiments can also be performed to interrogate 7 ip relaxation (cf. Sections 3.5 and 7.2.3) [Blii2]. If Tc denotes the correlation time of molecular motion then, from eqn (3.5.7) Tip can be expressed in the weak collision limit as... 

As two phases in quadrature will allow Carr-Purcell Meiboom-Gill, T, and or other spin-locking experiments, we recommend this as a minimum. However, providing different rf phases for the transmitter pulses is not difficult and this capability should not increase the cost of the spectrometer very much so you might as well go for four orthogonal phases. As discussed elsewhere, the phase shifts can be performed in the IF stage rather than at the carrier frequency so that one phase shifting network can suffice for all carrier frequencies. 

Proton relaxation under multiple pulse conditions has also been used to characterise phase composition in, for example, PET [95,96], and polyethylene [120]. The technique is particularly useful in the case of PET because the phases present generally do not show large differences in the decay times of their FID components, so FID analysis would be particularly problematic. Although the problem of assessing the extent to which spin diffusion is suppressed also applies to relaxation under multiple pulse conditions, the available experimental evidence suggests that they may be more effective in practice than the corresponding off-resonance spin-locking experiment [96]. This is almost certainly due to practical considerations rather than theoretical ones. 

Fig. 2. Behaviour of the magnetisation, M, in a spin-locking experiment to determine Tj...

The most important relaxation parameters are Ti, the spin-lattice relaxation time, T2, the spin-spin relaxation time, and the nuclear Overhauser enhancement, or NOE, where present. A second spin-lattice relaxation time. Tip, may also be measured in the rotating frame via spin-locking experiments (Section 2.4), and Ti itself may be subdivided into separate contributions as indicated above. Also the NOE depends upon which other nuclei are irradiated. 

Figure 6.13 Radio-frequency pulse sequences for (a) the S / CP NMR experiment, (b) a spin-lock experiment for direct determination of T,p and (d) a spin-lock experiment combined with CP for determination of the 7 p relaxation time, (c) Plot of the observed, transferred magnetisation M,(t) as a function of the CP contact time (t = Tcp) in Si H CP/MAS NMR spectra (7.1 T, = 4.0 kHz) of a mineral sample of kaolinite (circles) and a synthetic sample of a-dicalcium silicate hydrate (diamonds). The experiments employed yB /2]c ss ...

---