Preparing initial conditions for rotational resonance in solid state NMR spectroscopy

Abstract

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, ω A−ω X= nω r, where ω A and ω 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 σ( t=0)=Δ= S X z , regardless of the orientation of the crystallites. Preparation of such a state is a nontrivial task if the chemical shift anisotropy ω 0Δσ A,X of one (or both) of the sites is comparable to the difference in isotropic shifts Ω A−Ω X and hence to the required spinning speed ω r=(ω A−ω X)/ n. It is shown how the difference polarization state Δ can be prepared by combining total sideband suppression (TOSS) sequences with their time-reversed counterparts, or by more condensed sequences of rotor-synchronized π pulses.