A density matrix formulation of the theory of magnetic resonance spectra in slowly reorienting systems

When a spin system is coupled to a molecule or part of a molecule which is reorienting at a rate comparable to the width of the powder spectrum, the line shape can be calculated from a density matrix which is a function both of spin operators and of the Euler angles describing the molecular orientation. The problem of calculating such line shapes is eased by using the super-operator formalism and choosing symmetry adapted basis operators in the Liouville space of spins and orientations. Diffusion and strong collision models for molecular reorientation are discussed.