Inelastic neutron scattering study of the ``rotator'' phase transition in n -nonadecane

A simple kinematic model for rotational jump diffusion of a normal alkane about its long axis (circular random walk model) is developed. Inelastic neutron scattering data obtained on the Fermi chopper time‐of‐flight instrument at the National Bureau of Standards reactor using an incident neutron wavelength of $\text{2.47\hspace{0.17em}}\mathrm{\AA }\text{\hspace{0.17em}(Δ λ/λ ≃ 3.8 \%)}$ are compared with the predictions of the model. Data taken below the temperature of the ``rotator'' phase transition in n ‐nonadecane (295 °K) show no quasielastic scattering due to diffusive motions. Data taken in n ‐nonadecane in its disordered solid phase show quasielastic scattering consistent with the circular random walk model. Estimates for values of the model parameters of 3.5 psec. for ${\tau }_1$ and $\text{N ≥ 8}$ are obtained.