H/H fractionations in organic molecules: I. Experimental calibration of ab initio calculations

Carbon-bound hydrogen in sedimentary organic matter can undergo exchange over geologic timescales, altering its isotopic composition. Studies investigating the natural abundance distribution of H and H in such molecules must account for this exchange, which in turn requires quantitative knowledge regarding the endpoint of exchange, i.e., the equilibrium isotopic fractionation factor (aeq). To date, relevant data have been lacking for molecules larger than methane. Here we describe an experimental method to measure aeq for C-bound H positions adjacent to carbonyl group (Ha) in ketones. H at these positions equilibrates on a timescale of days as a result of keto-enol tautomerism, allowing equilibrium H/H distributions to be indirectly measured. Molecular vibrations for the same ketone molecules are then computed using Density Functional Theory at the B3LYP/6-311G** level and used to calculate aeq values for Ha. Comparison of experimental and computational results for six different straight and branched ketones yields a temperature-dependent linear calibration curve with slope = 1.081 0.00376T and intercept = 8.404 0.387T, where T is temperature in degrees Celsius. Since the dominant systematic error in the calculation (omission of anharmonicity) is of the same size for ketones and C-bound H in most other linear compounds, we propose that this calibration can be applied to analogous calculations for a wide variety of organic molecules with linear carbon skeletons for temperatures below 100 C. In a companion paper (Wang et al., 2009) we use this new calibration dataset to calculate the temperature-dependent equilibrium isotopic fractionation factors for a range of linear hydrocarbons, alcohols, ethers, ketones, esters and acids. 2009 Elsevier Ltd. All rights reserved.