Residual dipolar broadening

Based on random fields, relaxation theory R2 decreases as molecular tumbling gets faster and more effectively averages the residual dipolar broadening. Both in the fast and slow motion limits, R2 is proportional to tc. ... 

As will be shown later, the resolution provided by the combination of very-fast MAS (vr > 30 kHz) and high magnetic fields ( H Larmor frequency > 500 MHz) is sufficient to allow specific proton—proton proximities to be identified on the basis of the observation of particular peaks in H 2D DQ MAS spectra. The line narrowing achieved by MAS alone at a vr equal to 35 kHz is, however, still far from the limiting case, where all residual dipolar broadening has been removed. 

Homonuclear J coupling between abundant spins, such as F- F, or F- H can produce more intricate patterns, which are often obscured by residual dipolar broadening in spectra of polymer solutions (Sec. II.B). In solid-state NMR spectra, scalar couplings are not detected... 

The dipolar broadening parameter AG, independent of the type of solvent used, is related to the residual second moment (AM2b) ... 

Figure 14.1 ID spectra of a typical rubber, styrene-butadiene-rubber (SBR). a) Static spectrum acquired at a Larmor frequency of 500 MHz. The dipolar coupling is motionally averaged and different lines can be distinguished although they are still broadened by the residual dipolar couplings, b) MAS spectrum of the same sample at a MAS spinning frequency of 15 kHz. The line-broadening due to anisotropic spin interactions, e.g., residual dipolar couplings, is removed...

Since its first description in 1971 [35], gel-phase NMR was applied to peptide chemistry by Manatt and coworkers [36, 37], These authors used 13C NMR to determine the extent of chloromethylation of crosslinked polymers and 19F NMR to monitor protection-deprotection reactions. These two nuclei are the most commonly used in these types of studies, mainly because of their significant chemical shift dispersion, which can alleviate in part the resolution loss due to the non ideal linewidth obtained in the gel state. Apart from restricted molecular motion, that shortens T2 because of an efficient transverse relaxation, other sources of line-broadening derive from magnetic susceptibility variations within the sample (due to the physical heterogeneity of the system) and residual dipolar couplings. 

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