Pulse spin lock

Because of the favorable cross-peak multiplet fine-structure, the HSQC experiment offers superior spectral resolution over the HMQC (heteronuclear multiple quantum coherence) experiment [13, 14], On the other hand, the HMQC experiment works with fewer pulses and is thus less prone to pulse imperfections. The real advantage of the HSQC experiment is for measurements of samples at natural isotopic abundance and without the use of pulsed field gradients, since the HSQC experiment lends itself to purging with a spin-lock pulse. Spin-lock purging in the HMQC experiment... 

Rotating frame experiments where a multiple pulse spin-lock was used in the period. 

Figure 8.48. Practical mixing schemes for the ROESY experiment, (a) a single, low-power pulse, (b) a pulsed spin-lock comprising a repeated sequence of a small tip angle pulse followed by a short delay, and (c) the Tr-ROESY alternating-phase spin lock.

The application of various multi-pulse methods in NQR requires a clear theoretical understanding of physical processes first of all behind the simplest "basic" multi-pulse sequences which include such a popular sequence as multi-pulse spin locking—MW-4, and a sequence of identically spaced coherent radio frequency (RF) pulses with intervals between them less than the time constant of free induction decay (FID) —strong off-resonance comb— SQRC. ... 

The independence of Hgff on the time interval of the sequence under the condition that this interval lies between T2 and Tie can be used for determining proper Heff. This approach was first used in multi-pulse nuclear magnetic resonance in paper, and later, in NQR, for the analysis of quasi-stationary state in multi-pulse spin locking in the nuclear system with the spin of 5/tF at exact resonance. We extended this approach to the arbitrary offset of the pulse carrier frequency in relation to the resonance transition. 

It follows from (27) that for the multi-pulse spin-locking sequence ri2 = 0, so we shall obtain... 

The scheme of the experiment on polarization enhancement of NQR signals for PETN detection is shown in Figure 7. A PETN sample was placed for a fixed period of time between two identical ring magnets after which it was manually transferred inside the solenoidal coil of the detector of an NQR spectrometer where the signal from N nuclei was detected using a multi-pulse spin-locking sequence MW-4 (py — (t — - t) . 

Figure 8 Dependence of enhancement factor F on the time of protons polarization in magnetic field 6q 80 mT, measured for NQR signals received before and after polarization. NQR signal detection was performed with multi-pulse spin locking sequence (t — — t) with the...

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. 

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). 

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)... 

Total correlation spectroscopy (TOCSY) is similar to the COSY sequence in that it allows observation of contiguous spin systems [35]. However, the TOCSY experiment additionally will allow observation of up to about six coupled spins simultaneously (contiguous spin system). The basic sequence is similar to the COSY sequence with the exception of the last pulse, which is a spin-lock pulse train. The spin lock can be thought of as a number of homonuclear spin echoes placed very close to one another. The number of spin echoes is dependent on the amount of time one wants to apply the spin lock (typically 60 msec for small molecules). This sequence is extremely useful in the identification of spin systems. The TOCSY sequence can also be coupled to a hetero-nuclear correlation experiment as described later in this chapter. 

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.

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