Dynamics of crystalline acetanilide: Analysis using neutron scattering and computer simulation

Abstract

The unusual temperature dependence of several optical spectroscopic vibrational bands in crystalline acetanilide has been interpreted as providing evidence for dynamic localization. Here we examine the vibrational dynamics of crystalline acetanilide over a spectral range of ∼20–4000 cm−1 using incoherent neutron scattering experiments, phonon normal mode calculations and molecular dynamics simulations. A molecular mechanics energy function is parametrized and used to perform the normal modeanalyses in the full configurational space of the crystal i.e., including the intramolecular and intermolecular degrees of freedom. One‐ and multiphonon incoherent inelastic neutron scattering intensities are calculated from harmonic analyses in the first Brillouin zone and compared with the experimental data presented here. Phonon dispersion relations and mean‐square atomic displacements are derived from the harmonic model and compared with data derived from coherent inelastic neutron scattering and neutron and x‐ray diffraction. To examine the temperature effects on the vibrations the full, anharmonic potential function is used in molecular dynamics simulations of the crystal at 80, 140, and 300 K. Several, but not all, of the spectral features calculated from the molecular dynamics simulations exhibit temperature‐dependent behavior in agreement with experiment. The significance of the results for the interpretation of the optical spectroscopic results and possible improvements to the model are discussed.