Carbocátions e Geoquímica: em busca de novas correlações geoquímicas envolvendo diamantóides

Abstract

Recognized as the most stable tricyclic C10 isomers by George Olah, diamondoids are a class of rigid hydrocarbons, with high thermal and thermodynamic stability found only in petroleum whose formation process is very little known. The smallest representative of this family of cage-type hydrocarbons, adamantane, had its formation proposed from the rearrangement of carbocations during isomerization reactions in 1973 by Engler, Farcasiu and Schleyer. Since then, there are no works in the literature that seeks to elucidate the formation of this compound, even though it is configured as an important geochemical marker of oil. Thus, in this dissertation, the stability of the various C10H16 isomers was evaluated through quantum mechanics calculations and the various C10 H15+ cations with the aim of determining whether adamantane and the 1-adamantyl cation would be the so-called “thermodynamic wells” for its class of isomers. With these results, it was possible to infer about the formation of adamantane in oil as occurring through the rearrangement of carbocations of natural products, as well as proposing new ways of thinking about the formation of other C10 isomers of oil, especially naphthenes. For this, DFT calculations at the M06-2X/cc-pVTZ theory level were used to evaluate the stability of the different isomers and the AUTOMATON software used in the search for low-energy 1-adamantyl cation isomers. The calculations of the neutral isomers pointed to adamantane as the most stable C10H16 isomer among those known and cataloged on the NIST (National Institute of Standards and Technology) platform, as already expected due to the high symmetry of the molecule. However, the search for different low energy C10H15+ isomers with AUTOMATON pointed to the existence of an isomer, the 1-methyl-2,3,4,5,6,7-1H-indeno-1-ylium 12.7 kcal mol-1 more stable than the 1-adamantyl cation, indicating the existence of species more stable than this carbocation recognized until then as an example of stability. This species in its neutral and reduced form is configured as an important member of the family of petroleum naphthenes, which are bicyclic C10 hydrocarbons, whose formation 10 process is rethought in this work where a connection is proposed between them, adamantane and monoterpenes in petroleum.