Preparing initial conditions for rotational resonance in solid state NMR spectroscopy

The internuclear distance between two homonuclear dipolar-coupled dil. spins-1/2, SA and SX, can be measured by rotational resonance expts. in solid state NMR. These involve rotating the sample at the magic angle while fulfilling the condition for rotational resonance, WA-WX= nwr, where WA and WX are the isotropic shifts. Interpretation of the results is simplest if the spin system is prepd. in a state of pure difference polarization, described by an initial d. operator V(t = 0) = D = SAz-SXz, regardless of the orientation of the crystallites. Prepn. of such a state is a nontrivial task if the chem. shift anisotropy w0DVA,X of one (or both) of the sites is comparable to the difference in isotropic shifts WA-WX and hence to the required spinning speed wr = (WA-WX)/n. It is shown how the difference polarization state D can be prepd. by combining total sideband suppression (TOSS) sequences with their time-reversed counterparts, or by more condensed sequences of rotor-synchronized p pulses. [on SciFinder (R)]