H/H fractionation in organic molecules: III. Cyclic ketones and hydrocarbons

Quantitative interpretation of stable hydrogen isotope ratios (H/H) in organic compounds is greatly aided by knowledge of the relevant equilibrium fractionation factors (eeq). Previous efforts have combined experimental measurements and hybrid Density Functional Theory (DFT) calculations to accurately predict equilibrium fractionations in linear (acyclic) organic molecules (Wang et al., 2009a,b), but the calibration produced by that study is not applicable to cyclic compounds. Here we report experimental measurements of equilibrium H/H fractionation in six cyclic ketones, and use those data to evaluate DFT calculations of fractionation in diverse monocyclic and polycyclic compounds commonly found in sedimentary organic matter and petroleum. At 25, 50, and 75 C, the experimentally measured eeq values for secondary and tertiary Ha in isotopic equilibrium with water are in the ranges of 130& to 150& and +10& to 40& respectively. Measured data are similar to DFT calculations of eeq for axial Ha but not equatorial Ha. In tertiary Ca positions with methyl substituents, this can be understood as a result of the methyl group forcing Ha atoms into a dominantly axial position. For secondary Ca positions containing both axial and equatorial Ha atoms, we propose that axial Ha exchanges with water significantly faster than the equatorial Ha does, due to the hyperconjugation-stabilized transition state. Interconversion of axial and equatorial positions via ring flipping is much faster than isotopic exchange at either position, and as a result the steady-state isotopic composition of both H’s is strongly weighted toward that of axial Ha. Based on comparison with measured eeq values, a total uncertainty of 10–30& remains for theoretical eeq values. Using DFT, we systematically estimated the eeq values for individual H positions in various cyclic structures. By summing over all individual H positions, the molecular equilibrium fractionation was estimated to be 75& to 95& for steroids, 90& to 105& for hopanoids, and 65& to 100& for typical cycloparaffins between 0 and 100 C relative to water. These are distinct from the typical biosynthetic fractionations of 150& to 300&, but are similar to equilibrium fractionations for linear hydrocarbons (Wang et al., 2009b). Thus post-burial H exchange will generally remove the 50–100& biosynthetic fractionations between cyclic isoprenoid and n-alkyl lipid molecules, which can be used to evaluate the extent of H exchange in sedimentary organic matter and oils. 2013 Elsevier Ltd. All rights reserved.