Equilibrium ²H/¹H Fractionations in Organic Molecules

Citation

Wang, Ying (2010) Equilibrium ²H/¹H Fractionations in Organic Molecules. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/4MPX-MF10. https://resolver.caltech.edu/CaltechTHESIS:10212009-213942982

Abstract

Compound-specific H isotope analysis has become widespread over the past decade and stimulated a variety of studies using the H isotopic composition (δ²H values) of sedimentary organic molecules as paleoenvironmental proxies. Since alkyl H can be affected by a variety of exchange processes that lead to δ²H changes on geological timescales, interpretation of empirical δ²H data must account for these changes, which requires quantitative knowledge regarding the endpoint of the isotopic exchange, i.e., equilibrium fractionation factor (α_eq). Nevertheless, to date relevant data have been lacking for molecules larger than methane. This is because the conventional isotope exchange experiments suffer from the slow exchange rates of C-bound H (half-life ~ 10⁵–10⁶ years), whereas theoretical calculations — a convenient way to cover many organic structures over wide temperature ranges — are restricted by systematic biases for the H isotope system.

To remedy the situation, this project was proposed to use experimental equilibration data to calibrate ab initio calculations of α_eq. To accurately measure the value of α_eq within reasonable experimental timescale, I utilized the keto-enol tautomerism that leads to fast equilibration between H positions adjacent to carbonyl groups (denoted as H_α) and water. By equilibrating ketones with waters of varying δ²H values, the values of αeq were measured for Hα positions in a variety of acyclic and cyclic molecular structures at different temperatures. On the other hand, statistical thermodynamics and ab initio QM computations (B3LYP/6-311G**) were applied to calculate α_eq values for the same ketone molecules. Comparison between experimental and theoretical results yields a temperature-dependent linear calibration curve for linear molecules with slope = 1.081−0.00376T and intercept = 8.404−0.387T (T is temperature in degrees Celsius). For cyclic structures, the calibration is slightly different with slope of 1.44±0.05 and intercept of 32.8±5.1. Application of these calibration curves to more ab initio calculations generates the α_eq values for various H sites in alkanes, alkenes, ketones, carboxylic acids, esters, alcohols, and ethers, with the uncertainties estimated to be 10–25‰. 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(C=O)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. It is analogous to the well-known substituent effects in the aromatic ring system.

Our results provide a modular dataset to calculate equilibrium ²H/¹H fractionations for common molecules found in sediments and oils. By summing over individual H positions, the equilibrium fractionation relative to water between 0 and 100°C is estimated to be −70‰ to −90‰ for n-alkanes, around −100‰ for acyclic isoprenoids and −75 to −100‰ for steroids and hopanoids. The temperature dependence of these molecular fractionations is very weak within the relevant temperature range. The results agree well with field data for thermally mature hydrocarbons (δ²H values between -80‰ and -110‰ relative to water; Schimmelmann et al., 2006), suggesting that the observed δ²H changes in sedimentary organic matter can be confidently attributed to H exchange towards an equilibrium state.

Because of the need to accurately measure the widely-ranging δ²H values encountered in natural and isotopically-exchanged samples, a side project was conducted to quantitatively investigate the isotopic memory effects in compound-specific ²H/¹H analysis by gas chromatography/pyrolysis/isotope-ratio mass spectrometry (GC/P/IRMS), i.e., the situation in which the ²H/¹H ratio of a given chromatographic peak affects that of the following peak(s). Through a series of experiments that employed synthesized esters with δ²H varying by up to 1000‰, we were able to estimate the isotopic memory to be typically 2–4% of the nominal δ²H difference between two adjacent peaks. It increases with decreasing time separation, increasing analyte abundance of the preceding peak, or increasing age of the pyrolysis reactor. Roughly half of the memory effect can be attributed to the H2-adsorption process in the pyrolytic reactor, and the other half to unknown processes within the GC. Finally, based on our experimental and model study, modifications in routine analyses were proposed to mitigate memory effects.

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