Degradation of 5α-cholestane into dimethylperhydrophenanthrenes: an experimental and theoretical study |
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| Authors: | Mark A Sugden Adri C T van Duin Geoffrey D Abbott |
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| Institution: | Department of Civil Engineering and Geosciences, Drummond Building, University of Newcastle, Newcastle Upon Tyne NE1 7RU, UK |
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| Abstract: | A tetradeuteriated cholestane was heated with kerogen isolated from the Messel oil shale at 350 °C for 25 different time periods (ranging from 0.5 to 80 h) in borosilicate vessels. Three isomers of tetradeuteriated dimethylperhydrophenanthrene were formed and one of these was the major saturated hydrocarbon product under most conditions. The stereochemistry for these isomers was unknown but if the stable 5α,8β,9α(H),10β(CH3) sterane configuration is inherited from the cholestane then there are four possible isomers namely: I and II 13β(H) and 13α(H) diastereomers, respectively, of 10,13-dimethylperhydrophenanthrene] as well as III and IV 14β(H) and 14α(H) diastereomers, respectively, of 10,14-dimethylperhydrophenanthrene]. Molecular mechanics revealed that I is the most stable form and therefore is the component that dominated the saturated hydrocarbon degradation products. The identification of only three isomers of deuteriated dimethylperhydrophenanthrene in the pyrolysis experiments is supported by the calculated stabilities (ΔΔG) as isomer IV is considerably more unstable (by 1.7 kcal/mol) than II which is the least stable of I, II and III. If these compounds can be detected in crude oils their distributions may be useful maturity indicators in oils that have been generated at high thermal maturity. |
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