Cross-polarization CP

Figure 8 shows the 9si MASS and H cross polarization (CP MASS) spectra obtained on heated samples and 29Si MASS spectra collected after exposure of the 600 and 1100°C samples to water vapor. (Brinker, C. J., Kirkpatrick, R. J., Tallant, D. R., Bunker, B. C. and Montez, B., submitted.) The three prominent peaks at chemical shifts (6) of about -91, -101, and -110 ppm correspond to Q2, Q3, and Q4 silicon sites, respectively (44). The relative intensities of these peaks in the MASS spectra are proportional to the relative concentrations of the different silicon species. The positions of these peaks in both the MASS and CP MASS spectra are correlated with the average Si-O-Si bond angle, ij>, for bridging oxygens bound to the... 

These two techniques are called cross-polarization (CP) and magic angle spinning (MAS) in combination, these are thus called CP-MAS. 

The majority of double-resonance solid-state NMR experiments involving spin-1/2 nuclei use transfer of nuclear polarization via dipolar cross polarization (CP) to enhance polarization of the diluted spins S with small gyromagnetic ratio ys and significant longitudinal relaxation time T at the expense of abundant spins I with large y, and short 7 [215]. Typically, CP is used in combination with MAS, to eliminate the line broadening due to CS A, as well as with heteronuclear decoupling. To achieve the / S CP transfer, a (n/2)y pulse is applied at the I spin frequency,... 

This technique involves transfer of polarization from one NMR active nucleus to another [166-168]. Traditionally cross polarization (CP) was employed to transfer polarization from a more abundant nucleus (1) to a less abundant nucleus (S) for two reasons to enhance the signal intensity and to reduce the time needed to acquire spectrum of the less abundant nuclei [168]. Thus CP relies on the magnetization of I nuclei which is large compared to S nuclei. The short spin-lattice relaxation time of the most abundant nuclei (usually proton) compared to the long spin-lattice relaxation time of the less abundant nuclei, allows faster signal averaging (e.g., Si or C). CP is not quantitative as the intensity of S nuclei closer to 1 nuclei are selectively enhanced. Nowadays CP has been extended to other pairs of... 

We have seen how by magic-angle spinning (MAS) and high-power decoupling the resolution is rendered similar to that in the solution phase. The intensity of signals due to less abundant nuclei is enhanced by the use of cross-polarization (CP) pulse sequences. Excellent spectra of a variety of nuclei in the solid state have been obtained... 

For large-weight Zn + complexes with broad (50-150 kHz) second-order quadrupolar powder patterns Zn QE NMR may be an experimental challenge. In such cases the sensitivity must be enhanced by isotope enrichment combined with, e.g., cross polarization (CP) from H ", low-temperature acquisition or sampling of the free-induction decay (FID) in the presence of a train of refocusing pulses. ... 

These intermolecular correlation peaks mean that intermolecular cross polarization (CP) occurs between the carbon and proton of intermolecular -interacting polypeptides in blend. There may be two pathways for the observed intermolecular CP. One is that a direct transfer from proton to carbon exists, and another is that a change in the magnetization by spin diffusion (homonuclear Hartmann Hahn transfer) exists. It is thought that the former is much more efficient than the latter because the former comes from only one magnetization transfer process, but the latter comes from two... 

A suitable way to enhance NMR signals of nuclei with small magnetogyric ratios or low concentrations (rare spins 5) interacting with abundant spins I is the polarization transfer from the spin I to the spin S ensemble via a cross polarization (CP) experiment. This experiment is based on a double resonance technique, which can be applied in combination with MAS for the characterization of surface sites of solid catalysts and rigidly bound surface complexes. The pulse sequence used for CP experiments is demonstrated in Fig. 4. 

The cross-polarization (CP), i.e. the transfer of I-spin polarization to the dilute spins (S), is a double resonance experiment in which the I and S spins are coupled by a certain interaction, determined by the cross relaxation time tb. From the dynamics of the CP process, usually described with the spin temperature concept, the following equation for the time dependence of S-spin polarization could be derived ... 

Solid-state NMR suffers from two persistent problems (1) low abundance and/or sensitivity of the observed nucleus, and (2) long spin-lattice relaxation times Tx. Both can be remedied with the help of a double-resonance technique known as cross-polarization (CP) (21). 

This cross polarization (CP) was originally designed to enhance the sensitivity of nuclei with a low magnetogyric ratio (7) or a low natural abundance. 

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