Oxygen isotope equilibria of O-bearing organic compounds based on first principle quantum mechanical models, and implications for their use in the study of extraterrestrial organics
Surjyendu Bhattacharjee, John M. Eiler
Geochimica et Cosmochimica Acta, In Press, Journal Pre-proof, Available online 15 July 2024
“We report theoretically calculated equilibrium oxygen isotopic fractionation factors (17O/16O, 18O/16O) between a set of representative O-bearing organic molecules and water, as well as site specific 13C, 15N and 13C-18O equilibrium clumped isotopic anomalies in these compounds, all computed using density functional theory (DFT) methods combined with Urey-Bigeleisen-Mayer (UBM) calculations of reduced partition function ratios. We performed density functional theory (DFT) calculations with the B3LYP exchange correlation functional, and explored different basis sets, and treatments of solvation. After benchmarking results against prior theoretical and empirical studies, we conclude that B3LYP level of theory and aug-cc-pVTZ basis set with cluster solvation provides the most accurate treatment of this problem within the constraints of our approach. A representative set of O bearing organic compounds including aldehyde, ketones, amino acid and aromatic alcohol are predicted to be ∼24–41 ‰ higher in 18O/16O relative to water with θcompound – water varying in the range 0.522 – 0.526; and ∼ 23–41 ‰ lower in 13C/12C and ∼ 11 ‰ higher in 15N/14N relative to CO2 and N2, respectively (all presuming equilibrium partitioning) at 273 K.
This study is motivated by the study of soluble organic molecules found in carbonaceous chondrite meteorites, a significant fraction of which contain oxygen in their structure in the form of functional groups such as carbonyl, carboxylic acid, ester, ethers, and alcohol. These samples also contain oxygen-bearing macromolecular organic matter. We use the fractionation factors presented here to predict the triple oxygen isotope compositions of these organics, assuming equilibration with previously proposed early-solar-system volatile reservoirs and environments of organic synthesis.”
