Dipolar broadening parameter

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

Fig. 23. Plot of the dipolar broadening parameter AG (left scale) and its relation to the square root of rigid lattice second moment, AM2 = 15 kHz (right scale), for polystyrene networks crosslinked with DVB (solid symbols) and EDM (open symbols) swollen to equilibrium, vs l/n, the reciprocal nominal number of C—C bonds between crosslinks points. Solvents CC14 ( ), CDC13 (A),...

However, the Lorentzian form of the dipolar broadening function, which has the advantage of mathematical simplicity, is not suitable for an interpretation in terms of second moments it is replaced with a Gaussian dipolar function S(oa, AG), where the parameters AG correspond to the appropriate fractions of the square root of the intra-group rigid lattice second moments. With appropriate values for AG, calculated and experimental line shapes I(oo) are found to be in a good agreement for cross-linked polyethylene oxide) swollen in chloroform 1U). 

Fig. 2 A nitroxide scan on KcsA. (a) Linear representation of the putative transmembrane topology of KcsA and the nitroxide scan (linear scale with arrows), (b) Room temperature CW EPR spectra for two regions in TMl and TM2. Multiple nitroxide components are highlighted by red arrows in selected spectra, (c) Mobility and accessibility plots. Periodical pattern are visible. On the right, helical wheel representation showing the trends of the EPR parameters extracted from the spectra in a polar coordinate representation, (d) Example of dipolar broadening on position 108, and effect of underlabeling on the spectral shape. On the right the shortest distance...

The second moment of the dipolar-broadened NMR spectrum can be used as an alternative parameter for T2. It is in principle invariant with respect to nuclear motion in a solid, provided the statistical distribution of the spins remains unaltered. However, as the rate of motion increases with temperature, the spectrum narrows in the centre and wide skirts develop which become too weak for observation. In practice, therefore, the second moments of the recorded spectra decrease with increase in temperature. 

Fig. 4. Quadrupolar powder patterns (a) Spin NMR powder pattern showing that the central -)<- ) transition is broadened only by dipolar coupling, chemical shift anisotropy, and the second-order quadrupolar interactions, (b) Spin 1 NMR powder pattern for a nucleus in an axially symmetric electric field gradient (see text). The central doublet corresponds to 6 = 90° in Eq. (10). The other features of low intensity correspond to 6 = 0° and 6 = 180°. (c) Theoretical line shape of the ) - -) transition of a quadrupolar nuclear spin in a powder with fast magic-angle spinning for different values of the asymmetry parameter t (IS) ...

Thus, the occurrence of broad C resonances under H high-power decoupling and MAS conditions should neither lead to the automatic conclusion of lack of crystallinity of the sample, nor be considered a nuisance in contrast line broadening mechanisms under MAS are a valuable source of information for the extraction of kinetic parameters and activation energies of thermally activated processes. It should also be mentioned that further line broadening effects may arise from interference between ooi and the MAS frequency, leading to so-called rotary-resonance recoupling of dipolar... 

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