Dipolar broadening motional narrowing

For an effective relaxation process in a pure ID system, a second mechanism becomes more effective. This is the spin-spin dipolar interaction which is characterized by a specific rotation diagram (presence of a linewidth minimum at the magic angle). The effect of dimensionality on motional narrowing of this dipolar broadening has been extensively investigated in lowdimensional magnetic insulators, but this effect is not observed here for these molecular conductors [494]. 

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. 

Broad Lines. - The width of an n.m.r. line, At>1/2, is defined as the width in Hz at half signal height. Narrow lines, i.e., Ap1/2 < 10 Hz, are desirable in order to make use of chemical shift information and to follow chemical change. N.m.r. line widths in the liquid and the physisorbed state tend to be very narrow, with Ar>1/2 of the order of 10-1Hz. This fortuitous state arises because the molecular motion is sufficiently rapid and random in a liquid to average out the line broadening features present in solids, namely dipolar interactions, chemical shift anisotropy, quadrupolar interactions, and paramagnetic interactions which render the spectrum unusable under conventional or liquid-state experimental conditions. The mechanisms of each of these features will be described. The treatment will perforce be cursory, but an indication will be given to where a full theoretical treatment can be found. 

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