Simulated pressure response of crystalline indole.

The isostatic pressure response of crystalline indole up to 25 GPa was investigated through static geometry optimization using Tkatchenko-Scheffler dispersion-corrected density functional theory method. Different symmetries were identified in the structural evolution with increased pressure, but no motif transition was observed, owing to the stability of the herringbone (HB) motif for small polycyclic aromatic hydrocarbons. Hirshfeld surface analysis determined that there was an increase in the fraction of H···π and π···π contacts within the high pressure structures, while the fraction of H···H contacts was lowered via geometric rearrangements. It was found that isostatic pressure alone, up to 25 GPa, was not sufficient to induce a chemical reaction due to the poor π-orbital overlap existing within the HB motif. However, the applied pressure sets the stage for an activated chemical reaction when the molecules approach each other along the long molecular axis, with a reaction energy and reaction barrier of 1.05 eV and 1.80 eV per molecular unit, respectively.