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[en] Consideration is given to orientation modulation (om) of energy of weak nuclear quadrupole interaction by adiabatic crystal rotation around fixed axis at steady external static magnetic field. The main theoretical results were supported by experiments

[en] NMR orientation modulation (OM) in systems with a structureless singlet line, the center of gravity for which does not depend on the crystal orientation in the field, but the shape shows anisotropy in a crystal, coupled by multiparticle dipole-dipole interactions, is considered as an example

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[en] The electron-nuclear spin-spin interactions in solids give rise to the forbidden transitions (FT) in the magnetic resonance. This leads to important effects of two kinds: first, a change in the intensity of lines of the allowed transitions (AT) and in their position on the (field) frequency axis, second, the appearance of additional resonance lines belonging to the FT. Both effects are to a large extent determined by the character and intensity of the electron-nuclear interaction. Therefore, a comparison between the observed spectrum and the simulated one makes it possible to determine the parameters of the above interaction. The frequencies of the components of the FT and AT and their probabilities depend upon the orientation of the sample under study in the static field and on the direction and intensity of the variable field applied. The present work generalizes the theoretical and experimental results of studying the FT and their manifestation in the magnetic resonance. The behaviour of the spin-spin systems in the magnetic field is described within the framework of perturbation theory. The interaction Hamiltonian, which determines the character and regularities of the FT, is constructed for a contact interaction, a weak dipole interaction, a system of equally dipole-coupled spins, an anisotropic interaction, hyperfine and fine interactions, hyperfine and nuclear quadrupole interactions, forbidden transitions of free atoms and acoustic transitions. The paper describes some effects in the magnetic resonance associated with the FT, such as the forced dynamic polarization of nuclei, discrete saturation of EPR lines, cross relaxation, modulation of the amplitude of the electron spin echo. The author points out that the range of problems related to the FT effects is rather wide and is not yet sufficiently explored

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