Magic Angle Spinning

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Magic angle spinning (short: MAS) is a solid state NMR technique in which the sample is rotated very fast, this will average out dipolar interaction.

This spinning can be done at speeds up to 70 kHz, for the smallest sized rotors, the smaller the rotor is the faster it can spin

The angle of spinning to the magnetic field is chosen so that 3cos2Θ − 1 is equal to zero. Using another spinning angle it is also possible to cancel the second order quadrupolar interaction. Unfortunately it's not possible to spin the sample in two direction at the same time very fast. NMR spectra in solid state are usually very broad for materials that don't move by themselves, like for instance adamantane. Usually they have a linewidth that is around several hundreds of Hz for 15N. They can also be broader due to the chemical shift anisotropy and inhomogeneous orientation within the sample ( for membrane proteins for instance). When spinning the sample at low speed the chemical shift anisotropy will result in spinning sidebands. These spinning sidebands appear at distances to the main signal which are mutiples of the spinning frequency. At high enough speed the spinning sidebands get smaller.

Usually pulse sequences used to perform multidimensional NMR have to be synchronized with the rotor rotation.

The speed of the rotor can be accurately determined with an optical system, by spoting the tube with a highlighter. The inclination can be determined using a sample of KBr.

The proton dipolar interaction is too strong to be decoupled using rotation, so usually proton decoupling is also done during the experiment.

There are also experiments that allow to reintroduce the dipolar coupling during the rotation of the rotor ( like the REDOR experiment ).

see on wikipedia [[1]]

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