Effect of Structural Anisotropy in High-Pressure Reaction of Aniline

The pressure-induced reactivity of aromatic molecules in the crystal phase has been recently demonstrated to be a practicable route for the synthesis of crystalline nanothreads. The formation of these attracting materials is ascribed to stress anisotropy induced by uniaxial compression. In the case of aniline, the attainment of NH2-enriched carbon nanothreads above 30 GPa is related to the intrinsic crystal anisotropy due to the presence of strong directional H-bonds. Here, we have probed the reactivity of aniline crystal at lower pressure and higher temperature with the multifold purpose of testing the efficiency of H-bonds in ruling the crystal compressibility with increasing temperature; verifying the simple model based on the phonon assistance used in other cases to explain the crystal reactivity; and scaling down the synthesis of nanothreads. The reaction, monitored by infrared spectroscopy, is quantitative, but the product obtained is a hydrogenated graphitic carbon nitride. The simple model based on a reactivity driven by the lattice phonons is capable of accounting for the reactivity in the entire pressure range, whereas the difference in the products synthesized at pressures below and above 20 GPa is accounted for by the anisotropic compressibility of the unit cell due to the strong directional H-bonds. As a result, it is concluded that independent from the different products obtained, the high-pressure reactivity in aniline is always topochemical.