Rotational dynamics of propane in Na-Y zeolite: A molecular dynamics and quasielastic neutron-scattering study

We report results from molecular dynamics (MD) simulations and quasielastic neutron-scattering (QENS) measurements on the rotational dynamics of propane in Na-Y zeolite at room temperature with a loading of four molecules per $\alpha$ cage. Rotational part of the intermediate scattering function $F(Q,t)\mathrm{}$ obtained from the MD simulation suggests that rotational motion is faster relative to the translational motion. Various rotational models fitted to the MD data suggest that rotation is isotropic. It is found that the hydrogen atoms lie, on the average, on a sphere of radius $1.88\pm 0.05\AA ,$ which is also the average distance of the hydrogen atoms from the center of mass of the propane molecule. Results from QENS measurements are in excellent agreement with those obtained from MD, suggesting that the intermolecular potential employed in the MD simulation provides a realistic description of propane motion within faujasite. The rotational diffusion constant $D_R$ is $1.05\pm 0.09\times {10}^{12}{\sec }^{-1}$ from the QENS data, which may be compared with that obtained from the MD data $(0.82\mathrm{}\pm 0.05\times {10}^{12}{\sec }^{-1}).\mathrm{}$