Application of incoherent inelastic neutron scattering in pharmaceutical analysis: relaxation dynamics in phenacetin

This study centers on the use of inelastic neutron scattering as an alternative tool for physical characterization of solid pharmaceutical drugs. On the basis of such approach, relaxation processes in the pharmaceutical compound phenacetin (p-ethoxyacetanilide, C(10)H(13)NO(2)) were evidenced on heating between 2 and 300 K. By evaluating the mean-square displacement obtained from the elastic fixed window approach, using the neutron backscattering technique, a crossover of the molecular fluctuations between harmonic and nonharmonic dynamical regimes around 75 K was observed. From the temperature dependence of the quasi-elastic line-width, summed over the total Q range explored by the time-of-flight technique, it was possible to attribute the onset of this anharmonicity to methyl group rotations. Finally, using density functional theory-based methods, we were able to calculate the lattice vibrations in the harmonic approximation. The overall spectral profile of the calculated partial contributions to the generalized density of states compares satisfactorily to the experimental spectra in the region of the lattice modes where the intermolecular interactions are expected to play an important role. This study contributes to understanding the relationships between intermolecular hydrogen bonds, intramolecular dynamics, and conformational flexibility in pharmaceuticals on a molecular level, which can help in evaluating phase stability with respect to temperature variations on processing or on storage, and is related to control of polymorphism and pseudopolymorphism.