Friday, February 15, 2008
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.