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Position-Specific Hydrogen Isotope Equilibrium in Propane

Xie, Hao and Ponton, Camilo and Formolo, Michael J. and Lawson, Michael and Peterson, Brian K. and Lloyd, Max K. and Sessions, Alex L. and Eiler, John M. (2018) Position-Specific Hydrogen Isotope Equilibrium in Propane. Geochimica et Cosmochimica Acta, 238 . pp. 193-207. ISSN 0016-7037.

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Intramolecular isotope distributions can constrain source attribution, mechanisms of formation and destruction, and temperature-time histories of molecules. In this study, we explore the D/H fractionation between central (-CH_2-) and terminal (-CH_3) positions of propane (C_3H_8)- a percent level component of natural gases. The temperature dependenceof position-specific D/H fractionation of propane could potentially work as a geo-thermometer for natural gas systems, and a forensic identifier of specific thermogenic sources of atmospheric or aquatic emissions. Moreover, kinetically controlled departures from temperature dependent equilibrium might constrain mechanisms of thermogenic production, or provide indicators of biological or photochemical destruction. We developed a method to measure position-specific D/H differences of propane with high-resolution gas source mass spectrometry. We performed laboratory exchange experiments to study the exchange ratesfor both terminal and central positions, and used catalysts to drive the hydrogen isotopedistribution of propane to thermodynamic equilibrium. Experimental results demonstrate that D/H exchange between propane and water happens easily in the presence of either Pd catalyst or Ni catalyst. Exchange rates are similar between the two positions catalyzed by Pd. However, the central position exchanges 2.2 times faster than the terminal position in the presence of Ni catalyst. At 200 °C in the presence of Pd catalyst, the e-folding time of propane-water exchange is 20 days and of homogeneous exchange (i.e., equilibrium between central and terminal positions) is 28 min. An equilibrated (bracketed and time-invariant) intramolecular hydrogen isotope distribution was attained for propane at three temperatures, 30 °C, 100 °C and 200 °C; these data serve as an initial experimental calibration of a new position-specific thermometer with a temperature sensitivity of 0.25‰ per °C at 100 °C. We use this calibration to test the validity of prior published theoretical predictions. Comparison of data with models suggest the most sophisticated of these discrepant models (Webb and Miller, 2014) is most accurate; this conclusion implies that there is a combined experimental and theoretical foundation for an ‘absolute reference frame’ for position-specific H isotope analysis of propane, following principles previously used for clumped isotope analysis of CO_2, CH_4 and O_2 (Eiler and Schauble, 2004; Yeung et al., 2014; Stolper et al., 2014).

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Sessions, Alex L.0000-0001-6120-2763
Additional Information:© 2018 Elsevier Ltd. Received 25 January 2018, Revised 18 June 2018, Accepted 22 June 2018, Available online 2 July 2018. This research is supported by an NSF-EAR instruments and facilities grant and Caltech. Additional funding is provided by Exxon Mobil. We thank Michael Webb and Thomas Miller for helpful discussions and for providing the vibrational frequency calculations for the empirical PES. We thank Aaron Sattler and Michele Paccagnini for insightful advices on catalytic chemistry. We thank Nami Kitchen for advice and assistance with the experimental setup and the operation of the DFS mass spectrometer.
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Record Number:CaltechAUTHORS:20180702-111202770
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Official Citation:Hao Xie, Camilo Ponton, Michael J. Formolo, Michael Lawson, Brian K. Peterson, Max K. Lloyd, Alex L. Sessions, John M. Eiler, Position-specific hydrogen isotope equilibrium in propane, Geochimica et Cosmochimica Acta, Volume 238, 2018, Pages 193-207, ISSN 0016-7037, (
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:87518
Deposited By: Tony Diaz
Deposited On:03 Jul 2018 18:06
Last Modified:31 Jul 2018 22:02

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