Cross polarization, NMR

Prabakar, S., Wenslow, R.M., and Mueller, K.T. (2000) Structural properties of sodium phosphate glasses from Na -> cross-polarization NMR./. Non-Cryst. Solids, 263, 82-93. 

At 400 °C, the TGA (not shown) indicates a 20% mass loss which appears to result from depolymerization and redistribution of — [MeHSiO] — segments. The 29Si MAS and 111 NMR spectra suggest that the 400 °C sample is now composed solely of — [MeSi(0)i.5] —. However, the cross polarization NMR spectrum (not shown) reveals the presence of remnants of the — [MeHSiO] — and [MeSi(OH)0]— groups. The 13C spectrum changes somewhat as the 1.1 ppm peak is reduced to a shoulder on the major peak which shifts from —2.9 to —3.5 ppm. 

Collazo-Lopez H, Koch M, Cooper WT. 1993. Mathematical correction for variable T, m relaxation of overlapping signals in solid state 13C cross polarization NMR spectroscopy. Comput. Chem. 17 73-79. 

Morgan KR, Newman RH (1987) Estimation of the tannin content of eucalypts and other hardwoods by carbon-13 nuclear magnetic resonance Appita 40 450-454 Newman RH (1987) Effects of finite preparation pulse power on carbon-13 cross-polarization NMR spectra of heterogeneous samples J Magn Reson 72 337-340 Newman RH (1989) Carbon 13 NMR studies of lignin in solid samples - a review Chemistry Division Report, DSIR, New Zealand... 

Calculations of a from concentrations in the bulk solution indicate values of around 0.3% at the start of the induction period (B63,B56). These are likely to be minimum estimates, because they assume that no CH is formed in reality, small amounts may be, on account of concentration gradients, which will themselves introduce an error in the same direction. Reported values of a at the end of the induction period, summarized by Odier and Dorr (OlO), range from 0.1% to an improbable 8%. Their own estimate of 12% is supported by subsequent evidence from cross-polarization NMR (R16). Assuming a specific surface area of 300 m kg a value of 1% corresponds to a mean depth of attack of about 10 nm. 

Figure 5 Cross polarization NMR spectrum of organic macromolecular material from the Murchison (CM2) chondrite. Prominent peaks and shoulders are assigned to prohahle functional groups identified hy their respective chemical shifts in ppm (source Cody et al, 2002).

Table 7 Estimates of functional group abundance by cross-polarization NMR.

Yannoni et al. obtained MAS cross-polarization NMR spectra of the enriched 2-norbornyl cation in an Sbf s solid matrix at -196°C. The solid-state chemical shifts correlated well with the solution data except for the lack of resolution in the methylene region. Subsequently, they have been successful even in obtaining the NMR spectrum in the solid state at -268°C (5K) ... 

The combination of direct excitation or cross-polarization NMR experiments with off-MAS sample spinning seems to be the most powerful approach for the study of rotational diffusion of all system constituents. It allows for a chemical identification of all organic system components and simultaneously yields data on their rotational diffusion. Hence, for detailed studies on particle tumbling in complex systems, it represents the preferred experimental condition for all particle dispersions which are stable enough to survive the inertial field during sample spinning for an adequate period of time. ... 

Cross-polarization/magic-angle spinning and double cross-polarization NMR measurements... 

From previous discussions it is known that competing rate processes occur in the cross-polarization NMR experiment. So that the measured NMR signal truly represents the number of nuclei producing it, explicit measurements must be made for these various mechanisms, namely, TCf and 7jpn. For SPE experiments, 7j must be measured for the nucleus of interest. 

The chemical shift is a good indicator of the change in the carbon environment during the conversion from polymer to silicon carbide. When the pyrolysis temperature increases, there are fewer protons bonded to carbon, and therefore the chemical shifts move downfield [130] (formation of C=C double bonds). By this effect the conversion of aliphatic into aromatic carbon during pyrolysis is clearly indicated in the case of carbon-rich SiC. - cross polarization NMR experiments indicate that, even after heating to... 

Cross-polarization NMR relies on the presence of an abundant spin system ( H) to enhance the observation of a signal from a dilute spin system ( C). The idea is to transfer polarization (hence signal intensity) from the abundant H spins to the dilute C spins. The CP experiment consists of four basic timed sequences of radio-frequency (r.f.) pulses (Fig. 4). The left portions of Fig. 4 represent the experimental timing sequence and the right portions depict what happens to the nuclei in the sample under the action of the pulses. The four-part procedure consists of (1) polarization of the H spin system. 

Chemical Bond Labeling and Double Cross-Polarization NMR... 

Double-cross polarization nmr spectra were obtained using consecutive matched spin-lock transfers (J), first from H to and then from to The pulse sequence employed... 

Figure 3. Magic-angle cross-polarization NMR spectra of intact lyophiUzed soybean cotyledons grown in culture on media containing 4- C- N(amidefaspara-gine (left) atul N(amine)-asparagine (right). The major resotumce is due priinarily to peptide rutrogen and occurs 100 ppm downfield from solid ammonium sulfate...

In the double-label dilution experiment described here, the specificity of a particular metabolic pathway was examined by monitoring the fate of an existing labeled chemical bond In a molecule entering that pathway. Naturally, the complementary experiment can also be performed in which the formation of a labeled chemical bond from two separate specific labels Is observed directly using double-cross polarization nmr. 

We have Illustrated the use of double cross polarization nmr to follow the formation or metabolism of particular chemical bonds of proteins in Intact biological tissue. Neither the use of radioisotopes, nor the use of other means of detecting stable Isotopes, has comparable directness and specificity. 

Figure 5. Proton dipolar decoupled and cross polarized NMR spectra (24.3 MHz) at 25°C of poly(dAdT) poly(dAdT) fibers (right) and sal n sperm DNA fibers (left). The top spectra are from the fibers oriented perpendicular to the magnetic field in a teflon holder, and the bottom spectra are from parallel orientation. Humidity was controlled. (Reproduce, with permission, from Ref. 17.

Primary amines immobilized in the acidic montmorillonite (H+-mont) interlayer space, prepared by a silane-coupling reaction using 3-aminopropyltriethoxysilane, acted as base sites together with the interlayer acid sites to promote tandem deacetalization-Knoevenagel condensation (Scheme 6.22) [134]. On the basis of CP/MAS (CP, cross-polarization) NMR results, it is proposed that a free NH2 group in an expanded interlayer space of H+-mont in the presence of heptane... 

M. Hong, X. Yao, K. Jakes, D. Huster, Investigation of molecular motions by Lee-Goldburg cross-polarization NMR spectroscopy, J. Phys. Chem. B 106 (2002) 7355-7364. 

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