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

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. (CO)R or (CO)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 (δ²H values between −80‰ and −110‰ relative to water). Therefore the observed δ²H increase of individual hydrocarbons and the disappearance of the biosynthetic δ²H 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 δ²H values that are close to equilibrium with water. In these cases, constant down-core δ²H values for n-alkyl lipids cannot be reliably used to infer a lack of isotopic exchange.