Proton local dipolar field

Both spin-lattice (motional) and spin-spin processes contribute to TjpCC). Experimental cross-polarization transfer rates from protons in the local dipolar field to carbons in an applied rf field can be used to determine the relative contributions quantitatively. This measurement also requires a determination of the proton local field. Methods for making both measurements have been developed in the last few years [1,2]. For polystyrenes, the spin-lattice contribution to TjpCCO s is by far the larger. This means that the TipCCVs can be interpreted in terms of rotational motions in the low-to-mid-kHz frequency range. 

PISEMA experiments yield the local dipolar field experienced by the or nucleus. Perhaps counter intuitively, it has been shown that better resolution is obtained by using experiments which detect the local dipolar field on protons [129,... 

Figure 5.6 (a) Schematic diagram of the carbon and proton spin systems in a typical organic solid. Tc, Th and T are the spin temperatures of the carbons, protons and lattice. Ti and Tju are spin-lattice relaxation times in the rotating frame, laboratory frame and local dipolar field. Tch is the cross-polarization time. Efficient cross-polarization of dilute spins by the abundant spins requires 7 > Tj > fc > ch- ( ) Schematic diagram of the behaviour... 

Dipolar decoupling (DD) with high-powered proton decoupling can be used to coherently average the heteronuclear dipolar interactions to zero [12]. The DD forces the proton spins to change energy states at a fast rate compared to the frequency of the dipolar interactions. Under these circumstances, the local dipolar fields... 

The cross-polarization transfer of magnetization is inversely proportional to the sixth power of the C-H internuclear distance. Provided that the carbons in a polymer repeat unit are subjected to the same motions, carbons with directly bonded protons are expected to cross-polarize more rapidly than carbons without direct interactions because the shorter interaction distance results in a larger local dipolar field. Rapid molecular motion attenuates the cross-polarization mechanism. For protonated carbons in rigid solids, most of the signal buildup occurs over about the first 100 p,s. For nonprotonated carbons, most of the signal appears in the first few milliseconds. 

Three characteristics of the MRD profile change when the protein is hydrated with either H2O or D2O. Both terms of Eq. (6) are required to provide an accurate fit to the data. The second or perpendicular term dominates once the transverse modes become important. The power law for the MRD profile is retained, but the exponent takes values between 0.78 and 0.5 depending on the degree of hydration. A low frequency plateau is apparent for samples containing H2O which derives from two sources the field limitation of the local proton dipolar field as mentioned above, and from limitations in the magnetization transfer rates that may be a bottleneck in bringing the liquid spins into equilibrium with the solid spins. 

Carbon resonances arising from both nonprotonated and proto-nated aromatic carbons may appear at the same frequency under proton decoupling. Yet these two resonances could possess very different relaxation behavior and in a solid could evolve very differently due to local proton dipolar fields which attenuate with the carbon-proton distances as 1/rcH When the spin locking pulse for proton nuclei is turned off, carbons with directly bound protons such as methines and methylenes rapidly dephase in the local proton fields and their spectral response is rapidly diminished. The rapid internal motion of CH3 groups greatly decreases the effectiveness of methyl protons. Nonprotonated carbons are only dephased by remote and therefore... 

As we have indicated, solids generally give very broad, featureless NMR spectra whilst liquids may give very sharp lines, some times narrower. This would be true, for example, for the proton resonance in ice and liquid water at 273 K. Why is this The answer can only be in the fact that in the liquid the nuclear spins are undergoing much faster and more developed relative motions, i.e., rotations, translations, etc., and this must clearly lead to an averaging away of the dipolar proton-proton local fields responsible for the broad line in the ice. This indeed is the reason and we should then ask ourselves how fast do the motions have to be and about what directions in space must they occur in order that averaging will take place. 

Fig. 8.1. Proton-carbon dipolar coupling in an isolated C-H bond, (a) Lines of force from the proton magnetic moment (shown here in the + state, parallel to the external field Hq) generate a static local field at the C nucleus. Because Hu < Hq, the C experiences only Hf, the component of H that is antiparallel to Hq. (b) The resulting spectrum for a sample of isolated C-H ftagments with a single...

The proton rf field is much larger than the local proton-proton dipolar fields (rf signals in the 50-60-kHz range create a variation of only a few percent in polyethylene). 

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