From Methane to Nanodiamond Precursors in Water: Superacid-like Condensation Pathways Under Extreme Conditions

The chemical behavior of water and hydrocarbons under extreme pressures and temperatures lies at the heart of processes shaping planetary interiors, influences the deep carbon cycle, and underpins innovative high-temperature, high-pressure material synthesis. Recent experiments have shown that simple hydrocarbons immersed in water under extreme conditions transform into heavier hydrocarbons and nanodiamonds. However, the chemistry of water in these regimes, and its role in driving hydrocarbon condensation, remain poorly understood. Here, using atomistic simulations techniques, we show that water under extreme conditions acts like a strong superacid, protonating hydrocarbons and forming transient pentacoordinated carbocations such as CH₅ ⁺. These fleeting species can either transfer the proton to neighboring water species, or release molecular hydrogen to generate highly reactive carbocations that drive hydrocarbon chain growth. These mechanisms parallel the superacid catalyzed hydrocarbon condensation at ambient conditions that was discovered in the work of George Olah, who demonstrated that methane polycondensation proceeds via transient pentacoordinated ions in superacids. Our work shows that the same non-classical carbocation chemistry emerges in water under extreme conditions, leading to nanodiamond precursors. These findings reveal the existence of superacid-like hydrocarbon condensation in water, and provide a unifying reaction network that explains chemical transformations in environments such as planetary interiors.