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

Abstract

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 θ_{compound – 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.

Files

Additional details