C—H dipolar interaction

Since the C- H dipolar interaction depends on an intemuclear distance rCH as l/r H, the coupling for proton bearing carbons (r 1.1 A) is much greater as... 

Spinner assemblies (l.e., stator, holder, rotor) have been constructed from three materials Kel-F, Delrin, and machinable boron nitride (BN) (14). For observation of hydrocarbon materials at ambient and low temperatures, the Kel-F assembly is used. It displays suitable mechanical properties and does not interfere with the carbon spectrum since the resonances of the carbons in the Kel-F are >10 kHz in width due to the unremoved C-F dipolar interactions. To observe fluorocarbon materials at ambient and low temperature by C-F dipolar decoupling/CP/MAS, the Delrin assembly is used since unremoved C-H dipolar interactions broaden... 

Rotary resonance recoupling of heteronuclear C- H dipolar interactions in NMR spectra (recorded under conditions of H decoupling at frequency vi and MAS at frequency Vr) has been studied for three examples of molecular solids (adamantane, ferrocene and hexamethylbenzene) in which substantial molecular motion is known to occur. It has been shown that when rotary resonance conditions are satisfied (i.e. l Vr = , for n = 1 or 2), the recoupling can lead to motionally averaged Pake-like powder patterns from which information on C- H intemuclear distances and/or molecular motion can be derived. 

As shown in Fig. 3.1, NMR can also probe a wide range of frequencies for molecular motions which are reflected differently on various NMR parameters. In particular, spectra reflecting the chemical shift anisotropy (CSA), the C—H dipolar interaction, and the quadrupolar interaction are sensitive to the mid-range of frequencies, which are closely associated with important properties for glassy polymers such as impact strength and gas permeability. The basic equations and analyses for the and C CSA spectra are also described in this chapter. 

Fig. 7.8 Solid-state C NMR "powder pattern" line shape for the case of C— H dipolar interaction and an ensemble of randomly oriented C- H units. The frequency axis is...

If the proton spins could be driven to flip at a rate that is rapid compared to the static C- H dipolar interaction, which occurs naturally in mobile polymer solutions, then the resonance lines observed in solid-state NMR spectra would likewise no longer be broadened by these hetero-nuclear, spin-dipolar interactions. The NMR spectrum of PBT shown in Fig. 20.12(b) was recorded by applying an rf field B at the resonance frequency of protons, with a field strength of 50 kHz, in a direction perpendicular to the applied field Bo (analogous to the broadband H scalar-/... 

When the molecular tumbling is rapid, and the carbon relaxation is dominated by the C-H dipolar interaction, then... 

The CP rate is proportional to the dipolar second moment A/ch for the C resonance line due to the C-H dipolar interaction. 

Cross-polarization is more effective for rigid systems than for mobile structures. When extensive motion occurs, the C-H dipolar interactions are reduced, and this reduction causes a lower cross-polarization rate. In some systems, mobile components can exhibit sufficient motional anisotropy to permit cross-polarization because the anisotropy can impart a residual static component to the overall motion [69]. This appears to be the case for elastomeric systems. 

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