Dynamics of molecular reorientations: Analogies between quasielastic neutron scattering and deuteron NMR spin alignment

Abstract

The analogies between incoherent quasielastic neutron scattering and deuteron spin alignment in studying single particle reorientations are pointed out. Quasielastic neutron scattering measures the full van Hove correlation function for correlation times shorter than 10⁻⁸ s. Spin alignment represents an orientational single particle function for correlation times longer than 10⁻⁴ s. The analogy is based on the fact that geometrical information is obtained in both methods by manipulation of an appropriate phase factor. In neutron scattering this phase factor is Q⋅a in momentum space, where Q is the momentum transfer and a is a jump length. In spin alignment the phase factor is δ_Q⋅τ₁ in time space where δ_Q specifies the strength of the quadrupole coupling and τ₁ is a pulse spacing defining the evolution period. It is shown that the measurements of the elastic incoherent structure factor as a function of Q⋅R and the spin alignment echo height as a function of δ_Q⋅τ₁, respectively, yields largely equivalent information about the type of reorientational motion—although at different time scales. Therefore, both methods nicely supplement each other. Some examples (two‐, four‐, and six‐fold jumps of dimethylsulfone, hexamethylenetetramine, and hexamethylbenzene as well as rotational diffusion of polystyrene in its viscous state) are discussed explicitly, where the geometry and time scale of slow molecular reorientations are identified directly.