PREPARING INITIAL CONDITIONS FOR ROTATIONAL RESONANCE IN SOLID-STATE NMR-SPECTROSCOPY

The internuclear distance between two homonuclear dipolar-coupled dilute spins-1/2, S(A) and S(X), can be measured by rotational resonance experiments in solid state NMR. These involve rotating the sample at the magic angle while fulfilling the condition for rotational resonance, OMEGA(A)-OMEGA(X)=nomega*r), where OMEGA(A) and OMEGA(X) are the isotropic shifts. Interpretation of the results is simplest if the spin system is prepared in a state of pure difference polarization, described by an initial density operator sigma(t=0)=DELTA=S(z)A-S(z)X, regardless of the orientation of the crystallites. Preparation of such a state is a nontrivial task if the chemical shift anisotropy omega0DELTAsigma(A,X) of one (or both) of the sites is comparable to the difference in isotropic shifts OMEGA(A)-OMEGA(X) and hence to the required spinning speed omega(r)=(OMEGA(A)-OMEGA(X))/n. It is shown how the difference polarization state DELTA can be prepared by combining total sideband suppression (TOSS) sequences with their time-reversed counterparts, or by more condensed sequences of rotor-synchronized pi pulses.