Role of Spin Symmetry Conversion in Nuclear Relaxation in Solids

The rates of nuclear transitions in molecules which are able to rotate about a single axis in the solid are considered. Only the effects of intramolecular dipolar interactions are taken into account. In molecules with threefold or higher symmetry about the rotation axis, only transitions which alter the symmetry of the nuclear state can occur. In molecules with twofold symmetry, if the angle between the internuclear vector and the rotation axis is 90°, only transitions which retain the nuclear symmetry can occur. If that angle is not 90°, both nuclear transitions which alter and which conserve symmetry are allowed. The detailed mechanisms which cause these two types of nuclear transitions are similar.