Is it possible to predict the stability of a crystal structure under the influence of pressure? Quantum chemical study of ibuprofen crystals

Abstract

The method previously used to detect structural features of piracetam, which can be considered as preconditions for a crystal phase to crystal phase polymorphic transition, was applied to study the structure of ibuprofen, which was found to be stable under the influence of pressure. Analysis of the pairwise interaction energies between molecules has classified the ibuprofen structure as a layered packing of dimeric building units. The shear deformation modeling of strongly bound layers in the crystal structure under ambient pressure determined the [001] crystallographic direction along the (100) crystallographic plane as the only probable direction of crystal structure deformation. Under ambient pressure, the interaction energies between the mobile and fixed parts of the model system proved to be negative during the displacement in one crystallographic translation. The shift energy barrier of 10.1 kcal mol⁻¹ for such deformation was calculated using the B97D3/cc-PVDZ method. An increase in pressure results in the appearance of a region with positive interaction energies in the shift energy profile and a sharp increase in the shift energy barrier. In contrast to piracetam crystals, the increase in energy does not lead to the polymorphic transition of the ibuprofen crystals, but results in the appearance of additional C–H⋯π and C–H⋯O intermolecular interactions and their strengthening. This can be explained by the absence of a local minimum in energy near the starting point in the model system. The smallest displacement of the mobile part to the fixed one leads to a decrease in the interaction energy.