Studying Mechanical Properties and Phase Transitions of Aspirin Polymorphs with Terahertz Spectroscopy and ab Initio Simulations

Subtle structural differences and comparable lattice energies have resulted in controversial discussions on the relative stabilities, mechanical properties and further comparisons between two polymorphs of aspirin. In this study, terahertz time-domain spectroscopy (THz-TDS) is coupled with density functional theory (DFT) and ab initio molecular dynamics (AIMD) calculations to seek physical insight into the properties of crystalline aspirin. Large-size supercells incorporating defects are used to simulate disorder in the aspirin crystal structure to ultimately investigate possible mechanisms for polymorphic phase transformation. THz-TDS of aspirin single crystals is performed to investigate low frequency vibrational modes that may play a critical role in the phase transitions and mechanical properties of aspirin.