High-power heteronuclear decoupling

HPHD (1) High-power heteronuclear decoupling (2) High-power proton decoupling... 

In the case of pharmaceutical solids that are dominated by carbon and proton nuclei, the dipole-dipole interactions may be simplified. The carbon and proton nuclei may be perceived as dilute and abundant based upon then-isotopic natural abundance, respectively (Table 1). Homonuclear 13C—13C dipolar interactions essentially do not exist because of the low concentration of 13C nuclei (natural abundance of 1.1%). On the other hand, H—13C dipolar interactions contribute significantly to the broad resonances, but this heteronuclear interaction may be removed through simple high-power proton decoupling fields, similar to solution-phase techniques. 

Fig. 31. Stacked plot of the heteronuclear two-dimensional J-resolved spectrum of cured, carbon black filled, natural rubber. The proton flip experiment was used with high-power proton decoupling during the detection time. The experiment was performed with the sample spinning at the magic angle (reprinted from Ref. 1911 with permission)...

In general, the Hamilton operator H applicable to an experimentally observed resonance is the sum (3.1.1) of operators Hx of different interactions. The lineshape, therefore, is the result of all spin interactions. For observation of just one dominant interaction, special techniques need to be applied such as isotope enrichment, homonuclear multipulse and heteronuclear high-power decoupling. Nevertheless, in C NMR, for instance, despite high-power H decoupling an overlap of chemical shielding powder spectra centred at different chemical shifts is observed in most cases without site-specific isotope enrichment. 

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... 

Hence, originally designed to eliminate dipolar interactions, MAS has the main effect on the removal of the chemical shift anisotropy, which is rather pronounced in 13C NMR spectroscopy. Since heteronuclear interactions could be viewed as a perturbation of abundant spins (I) on the energy states of rare spin system (S), a more convenient way of reducing HD to zero is high power decoupling of the I nuclei < u = 0). 

The B lS field is turned off and free induction decay of spins is observed. During observation time, Bu is still on but serves as the high-power decoupling field to reduce heteronuclear I-S dipolar broadening. 

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