Setting the magic angle using single crystal sapphire rotors

Abstract

To date, the most common method for setting the magic angle in magic angle spinning (MAS) solid state NMR experiments has been to monitor the NMR signal from a quadrupolar nuclide (such as 79Br in KBr) under MAS, then swapping to a sample of interest. Here we introduce the use of single crystal sapphire rotors as a new method to set the magic angle while spinning a sample of interest. Using both simulations and experiment, we show that the 27Al satellite transitions for a single crystal sapphire rotor spinning about its crystalline c-axis manifest off magic angle as five resonances that coalesce into three peaks with maximum intensity at the magic angle. The difference in frequency between the satellite transitions can be used to measure the offset from the magic angle with high precision.