Abstract

The combined use of high-power proton decoupling and magic angle sample spinning (MAS) provides high-resolution C-13 NMR spectra for solids. In our laboratory we use these techniques to characterize polymeric materials. Magic angle sample spinning at high speeds reduces the intensity of first-order sidebands as the inverse square of the angular velocity of the rotor. The rotor speed depends on the design and the material of the spinner. Various rotor designs are being used for spinning at the magic angle (54.7°) with respect to the static magnetic field. These include the Andrew-Beams design and a cylindrical design with two bearings for both electromagnets and superconducting magnets. It has been noted that Delrin (polyoxymethylene) and Kel-F (polytrifluorochloroethylene) are suitable materials for the rotor. These rotors must be very stable and should have a long life for routine use. Our experience with Kel-F spinners shows that they wear quickly on the edges. The Delrin rotor spins faster and is stronger than the Kel-F rotor, but it also has a drawback in that the polyoxymethylene C-13 resonance appears at 89.1 ppm downfield from TMS in the NMR spectra. This resonance may interfere with the resonances of the solid polymers of interest. Slow speed may complicate the spectra with sidebands for some resonances. Here, we report some modifications of the Andrew-Beams-type spinner to overcome the above-mentioned difficulties. The modified spinners are stable and have a longer life compared with the Kel-F rotors. We also report a solid methylsilsesquioxane as a useful chemical shift reference compound.

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