Double Rotation (DOR) is a solid-state NMR method for obtaining a high resolution spectrum of half-integer spin quadrupolar nuclei. It was first described by Ago Samoson, Endel Lippmaa, and Alexander Pines (Mol. Phys., 65, 1013 (1988)). It is performed as a one-dimensional NMR experiment. In DOR, the sample is placed within a small rotor which is spinning inside a larger rotor that is also spinning, as shown in the figure below.
The spinning axis of the inner rotor is at an angle of 30.56° with respect to the spinning axis of the outer rotor. The spinning axis of the outer rotor is at an angle of 54.74° with respect to the external magnetic field. It is also necessary that the speed of the inner rotor be more than five times greater than the outer rotor, otherwise an anisotropic broadening could be re-introduced. Because of spinning speed limitations the isotropic resonances in DOR spectra are often flanked by sidebands, marked with asterisks in the figure above, which appear at integer multiples of the sum and difference of the inner and outer rotor frequencies. While the large number of sidebands represent the most serious limitation of DOR, it is, in principle, only a mechanical problem. The great advantages of DOR are its high sensitivity and ability to provide quick and quantitative spectra.
Study of the Aluminophosphates AlPO4-21 and AlPO4-25 by 27Al Double Rotation NMR,