Cross proton magnetization

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). 

Figure 3. Cross polarization magnetization for the PIP-cured epoxy under the SL (Hartmann-Hahn) condition. The cross polarization contact time is rcp. The decay corresponds to proton T,p relaxation.

The proton magnetic resonance (PMR) spectra of all parent compounds except the 2,7-isomer have been determined. All the spectra can be interpreted by first-order splitting rules including cross-ring and para spin-spin coupling. The various chemical shifts and coupling constants are listed in Table V. 

Clearly, in order to correctly apply CP pulse sequences for quantitative analysis (or even qualitative analysis), many relaxation processes (Tic, T pH, Till. TCp) must be considered and spectral acquisition parameters appropriately set. While a CP spectrum may be obtained when Tic > 7) 11 3TlpH Tqp, a quantitative CP spectrum requires that the recycle delay is sufficient (on the order of Tm) for the protons to be uniformly relaxed at the beginning of the contact time,25 all proton magnetization spin locked in the rotating frame decays at the same rate (T ph), and the contact time is sufficient to allow complete cross polarization (at least 5 times the longest TCp)26 Except when relative peak intensities are constant and appear to be correct, single contact time measurements should be avoided. Instead, 13C spectra and relaxation times should be measured and complete magnetization curves analyzed. 

Cross-polarization is a double-resonance experiment in which the energy levels of the H and 15N spins are matched to the Hartman-Hahn condition in the rotating frame [47], Under this condition the energy between the two coupled spin systems may be exchanged and the 15N magnetization (S) develops as a function of time at the expense of the proton magnetization according to the relation ... 

This effect is probably due to the structure of water in the gel, which is most probably different from the water structure within the coil. According to proton magnetic resonance measurements, the water within the gel in poly(styrene sulfonic acid) is less ordered than that outside the gel. Since the degree of order varies with the density of cross-linking, this discovery could also explain why the selectivity of an ionized gel with respect to ions goes through a maximum with increasing cross-link density. 

Cross, J.H., Gardian, D.G., Connelly, A. and Leonard, J.V. (1993) Proton magnetic resonance spectroscopy studies in lactic acidosis and mitochondrial disorders. J. In-her. Metab. Dis., 16, 800-811. 

Experimentally, the amount of sample required is determined by the sensitivity of the nucleus to be observed. Typically, mM of the sample (5-10 mg of lipid 0.5-1 mg of a small peptide 1-4 mg of a large protein) in a volume of about 0.7 mL or on 10-50 glass plates, may be needed for less sensitive ( H, i N) nuclei. For iR, 3ip and i C in MAS NMR, somewhat less material is required. Cross-polarization from the abundant proton magnetization may improve sensitivity, and decoupling (5-10 kHz for 80-100 kHz for H) is routinely used to improve spectral shape and reduce spectral widths. 

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