Pulse sequences cross-polarization

Throughout the cross-polarization pulse sequence, a number of competing relaxation processes are occurring simultaneously. The recognition and understanding of these relaxation processes are critical in order to apply CP pulse sequences for quantitative solid state NMR data acquisition or ascertaining molecular motions occurring in the solid state. 

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.

Fig. 10.25. Cross-polarization pulse sequences with H and F dipolar decoupling during data acquisition (a) — C cross-polarization (b) — H, followed by — C cross-...

Figure 2. Double cross-polarization pulse sequence. The y-axis shows radiofrequency field amplitude and K-axis time. A N direct difference signal can be accumulated by shifting the C radiofrequency off resonance for one scan and on resonance for the next. The second NMR signal is subtracted from the first. The time, T, during which the H is off is called the N hold time when the "C radiofrequency is off resonance, and is called the drain time when the C is on resonance. Data acquisition occurs with the H radiofrequency field turned back on for dipolar decoupling. Pulse sequence 1, hold Crf 60 kHz off resonance 2, W - C drain Crf on resonance 3, direct difference 1 minus 2.

One of the cross-polarization pulse sequences used to measure is shown in Figure 6.9. During the evolution time, At, of this pulse sequence, the magnetization is spin-locked along Any reorientation of the internuclear vectors induces fluctuations of the dipolar local fields, and the COcomponent of these fluctuating fields participates in the relaxation of the spins. This is a spin-lattice relaxation mechanism which is related... 

Figure 2. Multiple pulse sequences showing cross-polarization pulse and cross-polarization with protonated carbons suppressed. Used to observe the rigid components.

In addition to sample rotation, a particular solid state NMR experiment is further characterized by the pulse sequence used. As in solution NMR, a multitude of such sequences exist for solids many exploit through-space dipolar couplings for either signal enhancement, spectral assignment, interauclear distance determination or full correlation of the spectra of different nuclei. The most commonly applied solid state NMR experiments are concerned with the measurement of spectra in which intensities relate to the numbers of spins in different environments and the resonance frequencies are dominated by isotropic chemical shifts, much like NMR spectra of solutions. Even so, there is considerable room for useful elaboration the observed signal may be obtained by direct excitation, cross polarization from other nuclei or other means, and irradiation may be applied during observation or in echo periods prior to... 

Figure 2. Pulse sequence for 13C-serve REDOR NMR. This sequence differs from the original REDOR pulse sequence (ref. 18) in that n pulses alternate between 13C and 15N r.f channels. On alternate scans of the REDOR experiment, the 15N iz pulses are either applied or omitted. This figure illustrates that the REDOR pulse sequence with four rotor periods of 13C-15N dipolar-coupling evolution (Nc = 4) NC can be increased (in increments of two) by adding rotor periods and pairs of 13C and 15N n pulses between the end of the cross-polarization preparation and the start of data acquisition.

The combination of cross polarization (basically a pulse sequence) and MAS is sufficient to drastically reduce the linewidths of spin-Vi nuclei. Liquid-state proton NMR spectra, as we have seen, are characterized by extremely narrow lines and complex multiplets due to spin-spin coupling in addition, the normal chemical shift range is only around 10 ppm. 

Figure 1. Schematic diagram of the solid-state NMR pulse sequences for (a) quantitative single pulse 13C observe with gated decoupling and (b) Ti and (c) 13C Ti determinations via cross polarization.

By contrast, in the high field of a superconducting magnet, polarization transfer to heteronuciei is less efficient, because now the difference in resonance frequency of XH and 13C is significant and exceeds the magnitude of the coupling constants between the carbons and the protons. In this latter case, polarization transfer can be achieved most effectively by appropriate pulse sequences (e.g., via cross-polarization). 

Fig. 4. A schematic diagram of the 2D PDLF (PELF) method using the BLEW-48 sequence for 1H-1H dipolar decoupling. CP denotes cross polarization. The last two jt/2 pulses of the S spin act as a Z filter. (Reproduced by permission of Elsevier Sicence.)...

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