Confirmation of Phase Transitions and Laser-Assisted Chemical Reaction for Pyridine under High Pressure

The high-pressure behavior of pyridine has been controversial for many years. In the current paper, Raman and infrared spectra under high pressure proposed a liquid-to-solid transition at about 1 GPa, followed by solid-to-solid transitions at about 2.5, 10.9, and 17.1 GPa. The synchrotron high-pressure XRD patterns of pyridine confirmed these phase transitions. On one-way loading up to 50 GPa, no other new phase is observed. With laser-assisted irradiation, pressure-induced reactions can occur, and the pressure threshold of the chemical reaction of pyridine is reduced to 1.5 GPa, less than 23 GPa from the single compression at room temperature. After release of the various solid phases of pyridine under high pressure to ambient conditions, two distinct chemical reaction products are obtained. Carbon nitride nanothreads and amorphous forms are obtained by slow and fast decompression, respectively. The experimental results reveal that only a laser with a suitable wavelength can induce chemical reactions, regardless of power. The laser-induced reactions were not chain reactions. Liquid chromatography–mass spectrometry of the products suggested that pyridine first opens the ring through the C–N bond being broken and then transforms into the products that probably contained molecules like 1-allylpyridinium nitrides and its multiple polymers.