Evaluation of Shear-Slip Transitions in Crystalline Aspirin by Density-Functional Theory

Crystalline aspirin has been shown to form two distinct polytypes, or polymorphs, that differ only in one unit-cell dimension. The second polytype is metastable and has been proposed to convert to the original form through a one-dimensional shear-slip mechanism. The feasibility of the {100}< 001 > shear-slip system, relating the two known aspirin forms, is examined computationally by the use of periodic-boundary density-functional theory (B86bPBE-XDM). A low barrier of ca. 10 kJ/mol per molecule is computed, which is estimated to be consistent with the observed interconversion rate, accounting for uncertainties in the treatment of thermal effects. The barrier is shown to increase under applied pressure, explaining previous experimental observations that compression of aspirin-II did not result in reversion to aspirin-I under the time scales considered. Finally, the advisability of using aspirin, or other compounds that form similar polytypes, as tests of computational methods for studies of polymorphism is discussed. Because of their high geometric similarity, both polytypes are predicted to be effectively degenerate regardless of the treatment of intermolecular dispersion.