Computational Investigation of the Stability of Di-p-Tolyl Disulfide "Hidden" and "Conventional" Polymorphs at High Pressures

The investigation of molecular crystals at high pressure is a sought-after trend in crystallography, pharmaceutics, solid state chemistry, and materials sciences. The di-p-tolyl disulfide (CH3-C6H4-S-)(2) system is a bright example of high-pressure polymorphism. It contains "conventional" solid-solid transition and a "hidden" form which may be obtained only from solution at elevated pressure. In this work, we apply force field and periodic DFT computational techniques to evaluate the thermodynamic stability of three di-p-tolyl disulfide polymorphs as a function of pressure. Theoretical pressures and driving forces for polymorphic transitions are defined, showing that the compressibility of the gamma phase is the key point for higher stability at elevated pressures. Transition state energies are also estimated for alpha -> beta and alpha -> gamma transitions from thermodynamic characteristics of crystal structures, not exceeding 5 kJ/mol. The beta -> gamma transition does not occur experimentally in the 0.0-2.8 GPa pressure range because transition state energy is greater than 18 kJ/mol. Relations between free Gibbs energy (in assumption of enthalpy) of phases alpha, beta, and gamma, as a function of pressure, are suggested to supplement and refine experimental data. A brief discussion of the computational techniques used for high-pressure phase transitions is provided.