# The Adamantaneland Revisited

## TL;DR

This study investigates how diamondoids like adamantane form, revealing that certain carbocations are more stable than expected, which could help explain their geological origins.

## Contribution

The paper identifies new stable carbocation structures linked to diamondoid formation, offering insights into their geochemical pathways.

## Key findings

- Adamantane is the most stable C10H16 isomer, but its carbocation is not the most stable.
- Allylic carbocations with 1-methylhexahydroindene are more stable and may be connected to diamondoid formation.
- These findings suggest a geochemical link between naphthenic hydrocarbons and diamondoids.

## Abstract

Diamondoids are a
class of rigid, cage-like hydrocarbons found
exclusively in petroleum on Earth, renowned for their exceptional
thermal and thermodynamic stability. Their resistance to decomposition
under geological conditions makes them valuable as geological markers.
However, a limited understanding of the processes leading to their
formation has hindered their broader application, particularly in
comparison to conventional biomarkers. This study explores the formation
pathways of the simplest diamondoid, adamantane, via a carbocationic
mechanism originating from isomeric hydrocarbons. The thermodynamic
stability of adamantane and the 1-adamantyl cation was assessed relative
to their isomers using the M06–2X/cc-pVTZ level of theory.
The results confirm that adamantane is the most stable C10H16 isomer; however, its corresponding carbocation, 1-adamantyl,
is not the most stable C10H15
+ species.
Instead, the most stable carbocations are allylic species containing
the 1-methylhexahydroindene ring system, which is associated with
the naphthenic fraction of petroleum. These findings suggest that
hydrocarbons containing the 1-methylhexahydroindene ring system may
have a geochemically relevant connection to diamondoid formation,
and their ratios can be of geochemical interest.