Dynamic-angle spinning of quadrupolar nuclei

In dynamic-angle spinning (DAS), a sample spins around an axis inclined at an angle θ(t) with respect to the magnetic field such that the averages of Pn(cosθ) are zero. The simplest case is where θ(t) assumes two discrete values θ1 and θ2 (complementary DAS angles) such that the averages of P2(cosθ) and P4(cosθ) are zero, thereby removing second-order quadrupolar (and dipolar) broadening. Examples of DAS complementary angles are θ1=37.38° and θ2=79.18°. Experimental details for DAS experiments are provided and applications to sodium-23 and oxygen-17 NMR illustrate the enhanced resolution achieved by removing the second-order broadening inherent in magic-angle spinning.