The Adamantaneland Revisited

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.