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

## TL;DR

This paper shows how water under extreme conditions behaves like a superacid, enabling methane to form nanodiamond precursors through unique chemical reactions.

## Contribution

The paper reveals that water under extreme conditions can act as a superacid, enabling carbocation-driven hydrocarbon condensation similar to classical superacid chemistry.

## Key findings

- Water under extreme conditions protonates methane to form transient pentacoordinated carbocations like CH5+.
- These reactive species drive hydrocarbon chain growth and nanodiamond precursor formation.
- The process mirrors superacid-catalyzed reactions discovered by George Olah.

## Abstract

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.

Under extreme pressure and temperature, superionic water acts as a strong superacid, catalyzing methane polycondensation through pentacoordinated carbonium ions (CH5
+). This discovery reveals the non‐classical carbocation chemistry underlying the early stages of diamond formation in water under extreme conditions.