Wednesday, November 21, 2007
Magic Angle Spinning
The NMR spectra of solids generally have very broad lines due to a number of interactions which are averaged either to zero or an isotropic value in solution. It is the averaging of these interactions which leads to the narrow lines observed in liquids. The rapid random motion responsible for the averaging in solution is not present for solids, however, it can be mimicked by spinning the sample about an angle of 54.7 degrees with respect to the magnetic field at a rate comparable to or greater than the extent of the interaction being averaged. This technique is called magic angle spinning (MAS) and it allows the measurement of high resolution spectra of solids. When the spinning rate is less than the extent of the interaction, the spectrum is split into sidebands spaced at the spinning speed. In the figure below are 81 MHz 31P CPMAS spectra of ammonium dihydrogen phosphate with high power proton decoupling at various spinning rates. When the spinning speed is zero, one can see the powder spectrum characterizing the chemical shift interaction. As the spinning speed is increased, there is a centerband at the isotropic chemical shift with sidebands whose intensities can be used to calculate the chemical shift tensor. When the spinning speed exceeds the span of the chemical shift interaction the sidebands are very small and a liquid-like spectrum is obtained.