Setting the Magic Angle

High resolution solid state NMR employs a technique called "Magic Angle Spinning" where the sample is spun at an angle of 54.736 degrees with respect to the magnetic field at a rate fast with respect to the interactions being averaged. In order to achieve high resolution, the angle must be set very precisely. This is commonly done by looking at the ⁷⁹Br signal of KBr. The intensity of the spinning sidebands for the satellite transition of the I = 3/2 ⁷⁹Br is strongly related to the precision of the magic angle. The sidebands appear as rotational echos in the ⁷⁹Br FID. The more rotational echoes - the stronger the sidebands and therefore the more precise the angle setting. Although any quadrupolar isotope with strong satellite transition sidebands can be used for this purpose, the ⁷⁹Br of KBr is particularly convenient as the resonance is very close to ¹³C, has a short T₁, and can be seen easily in one scan. The spectrometer is set up to scan without adding the signals and the angle adjustment is made while observing the FID until a maximum number or rotational echos is observed.