Deuterium Isotope Effects in Polymerization of Benzene under Pressure

The enormous versatility in the properties of carbon materials depends on the content of the sp² and sp³ covalent bonds. Under compression, if intermolecular distances cross a critical threshold, then unsaturated hydrocarbons gradually transform to saturated carbon polymers. However, the mechanism of polymerization, even for benzene, the simplest aromatic hydrocarbon, is still not understood. We used high-pressure synchrotron X-ray, neutron diffraction, and micro-Raman spectroscopy together with density functional calculations to investigate the isotope effects in benzene isotopologues C₆H₆ and C₆D₆ up to 46.0 GPa. Raman spectra of polymeric products recovered from comparable pressures show the progression of polymerization exhibiting a pronounced kinetic isotope effect. Kinetically retarded reactions in C₆D₆ shed light on the mechanism of polymerization of benzene. We find that C₆D₆-derived products recovered from P < 35 GPa actively react with moisture, forming polymers with higher sp³ hydrogen contents. Significant isotopic shift (≥7 GPa) in persistence of Bragg reflections of C₆D₆ is observed.