High-power dipolar decoupling

Fig. 7. Nmr spectra of quinine [103-95-0] C2QH24N2O2, acquired on a Bruker 300AMX spectrometer using a Bmker broadband CP/MAS probe, (a) Proton-decoupled spectmm of quinine in CDCl (b) the corresponding spectmm of solid quinine under CP/MAS conditions using high power dipolar decoupling (c) soHd-state spectmm using only MAS and dipolar decoupling, but without cross-polarization and (d) soHd quinine mn using the...

Fig. 6.6.4. Dipolar dephasing pulse sequence for discriminating in favour of amorphous (mobile) domains. [CT = contact time DD = high-power (dipolar) decoupling ACQ = acquisition Vr = MAS rate].

Fig. 6.6.5. Pulse sequences for measuring proton and fluorine relaxation times selectively for different domains in a semicrystalline polymer, (a) The Torchia method as modified for Tf. (b) Sequence for Tfp. (c) (See next page) Pre-CP proton spin inversion for T . (d) (See next page) Pre-CP proton spin-locking for T . For (c) acquisitions from the two component sequences add (upper sequence) and subtract (lower sequence) to give a Freeman-Hill type of measurement. For measurement of Tf by sequence 5(d) the time t is varied. However, when this sequence is used to discriminate in favour of crystalline (relatively rigid) domains, it is fixed at a value between those of Tf for the crystalline and rigid domains. [CT = contact time SL = spin-lock time DD = high-power (dipolar) decoupling ACQ = acquisition.]...

The spin-lattice relaxation time for in solids is very long (several minutes). Since the nuclei have to relax before another excitation pulse can be sent, this requires hours of instrument time in order to collect a spectrum of reasonable intensity. A pulse technique called cross-polarization can be used to reduce this effect by having the protons interact with the carbon nuclei, causing them to relax more rapidly. FTNMR systems for sohd samples include the hardware and software to produce narrow line spectra from solid samples in a reasonable amount of time using high-power dipolar decoupling, MAS, and cross-polarization. 

Figure 5.1 CNMR spectra of bulk poly(butylene terephthalate) obtained using lower-power decoupling (a), high-power (dipolar) decoupling (b), and dipolar decoupling with rapid magic-angle sample spinning (c). (Reprinted with permission from Jelinski [2].)...

As first suggested and demonstrated by Schaefer and Stejskel [13], combination of the techniques of high-power dipolar decoupling (DD), rapid magic-angle spinning (MAS) [7], and cross-polarization (CP) [11] of and C nuclear spins permits observation of high-resolution C NMR spectra... 

In most molecules, there are direct and indirect couplings between the C and other spins such as H. When the shift of a resonance line is the result of two tensorial interactions, the problem becomes more complicated. For polymers, the C chemical-shift interactions can be isolated from the dipolar interactions by removal of the proton interaction with high-powered dipolar decoupling. 

---