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

Equilibrium H/H fractionation factors (aeq) 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 Ha in ketones (Wang et al., 2009). The total uncertainty in reported aeq values is estimated at 10–20&. The effects of functional groups were found to increase the value of aeq for H next to electron-donating groups, e.g. AOR, AOH or AO(C@O)R, and to decrease the value of aeq for H next to electron-withdrawing groups, e.g. A(C@O)R or A(C@O)OR. Smaller but significant functional group effects are also observed for Hb and sometimes Hc. 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 aeq agree well with field data for thermally mature hydrocarbons (d H values between 80& and 110& relative to water). Therefore the observed dH increase of individual hydrocarbons and the disappearance of the biosynthetic dH 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 dH values that are close to equilibrium with water. In these cases, constant down-core dH values for n-alkyl lipids cannot be reliably used to infer a lack of isotopic exchange. 2009 Elsevier Ltd. All rights reserved.