Insights into the structure and polymerization mechanisms of CO molecules under pressure

High pressure technique can greatly enrich the chemistry research by innovating the traditional research paradigm. Recently, tremendous attentions have been paid to the high-pressure behavior of low-Z molecules, such as CO, CO₂, N₂, O₂ and mixtures. These molecules tend to polymerize into extended solids at the pressure of 1–100 GPa, but the structures and polymerization mechanisms are still poorly understood. Herein, as a research model, high pressure polymerization process of carbon monoxide (CO) is studied in detail both experimentally and theoretically. The in-situ Raman spectra and angle-resolved X-ray diffraction experiments prove the successful synthesis of p-CO and its amorphous structure. The theoretical simulations reveal that two CO molecules dimerize into the ethylenedione (OCCO) diradical with spin-polarized singlet state firstly, then the OCCO diradical induces the subsequent chain elongation, ring closure and chain crosslinking reactions, leading to formation of the amorphous 3D network. The multiple basic units, hybrid coordination of C/O atoms and complex connecting styles in p-CO are revealed. Based on the polymerization mechanisms, the fundamental principles governing the character (amorphous or crystalline) of extended solids under high pressure are elucidated. Due to the small dipole moment and the head-to-tail disorder of CO molecules, it is reasonable to speculate that crystalline p-CO may exist under more rigorous conditions than 110 GPa and 2000 K, at which the isoelectronic nitrogen (N₂) molecules polymerize into a single-bonded cubic form of nitrogen. Our study provides a profound insight into the polymerization mechanism and structures of low-Z CO molecules under compression, contributes to the diversified chemical researches and has a generally scientific implications for the interior dynamics of planets.