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Published December 1, 2009 | Supplemental Material
Journal Article Open

Equilibrium ^2H/^1H fractionations in organic molecules. II: Linear alkanes, alkenes, ketones, carboxylic acids, esters, alcohols and ethers

Abstract

Equilibrium ^2H/^1H fractionation factors (α_(eq)) for various H positions in alkanes, alkenes, ketones, carboxylic acids, esters, alcohols, and ethers were calculated between 0 and 100 °C using vibrational frequencies from ab initio QM calculations (B3LYP/6-311G**). Results were then corrected using a temperature-dependent linear calibration curve based on experimental data for H_α in ketones (Wang et al., 2009). The total uncertainty in reported α_(eq) values is estimated at 10–20‰. The effects of functional groups were found to increase the value of α_(eq) for H next to electron-donating groups, e.g. -OR, -OH or -O(CO)R, and to decrease the value of α_(eq) for H next to electron-withdrawing groups, e.g. -(C=O)R or -(C=O)OR. Smaller but significant functional group effects are also observed for H_β and sometimes H_γ. By summing over individual H positions, we estimate the equilibrium fractionation relative to water to be −90‰ to −70‰ for n-alkanes and around −100‰ for pristane and phytane. The temperature dependence of these fractionations is very weak between 0 and 100 °C. Our estimates of α_(eq) agree well with field data for thermally mature hydrocarbons (δ^2H values between −80‰ and −110‰ relative to water). Therefore the observed δ2H increase of individual hydrocarbons and the disappearance of the biosynthetic δ^2H offset between n-alkyl and linear isoprenoid lipids during maturation of organic matter can be confidently attributed to H exchange towards an equilibrium state. Our results also indicate that many n-alkyl lipids are biosynthesized with δ^2H values that are close to equilibrium with water. In these cases, constant down-core δ^2H values for n-alkyl lipids cannot be reliably used to infer a lack of isotopic exchange.

Additional Information

© 2009 Elsevier Ltd. Received 9 March 2009; accepted 20 August 2009. Available online 23 August 2009. Associate editor: James Kubicki. We acknowledge Adri van Duin, Edwin A. Schauble and Weifu Guo for helpful discussions of ab initio calculations. This work is supported by Petroleum Research Fund (PRF) of the American Chemical Society (ACS), #43746-G2 and by National Science Foundation (NSF), Grant #EAR-0645502.

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