The effect of macroscopic rotation on anistotropic bilinear spin interactions in solids

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Abstract
An examination is made of the effect of rapid specimen rotation on the N.M.R. spectra of solid specimens, with particular reference to the anisotropic electron-coupled spin-spin interaction. It is shown that when the specimen is rotated rapidly about an axis inclined at the special angle of 54° 44′ to the direction of the external magnetic field the spectral broadening arising from any anisotropy of the electron-coupled spin-spin interactions is removed provided the coupling tensors are symmetric. The dipolar interactions and the anisotropy of the chemical shift tensors are of course removed also, and the spectra are in such cases determined by the scalar shifts σ and coupling constants J as in isotropic fluids. It is however found that when a spin-coupling tensor has an antisymmetric component, this component is not in general removed by the rapid rotation. The spectrum is then determined by antisymmetric coupling constants A in addition to σ and J. A discussion is given of the role played by the constant A in the determination of the N.M.R. spectra.