Exploring clumped isotope fractionation of organic molecules using density functional theory

Abstract

The measurement of doubly-substituted (‘clumped’) isotopologues provides important information regarding the source, sink, and potentially the temperature of formation of a given molecule. While currently limited to small molecules such as methane or carbonate, recent technological developments are expected to expand the range of molecules studied, paving the way for clumped isotope measurements in various organic compounds. Theoretical calculations, including those based on density functional theory (DFT), can serve as guidelines for future methodological developments. However, these calculations have so far been reported for a small number of organic molecules and clumping types.

Here, we report DFT calculations for DD, ¹³CD, ¹³C¹³C, ¹³C¹⁵N, and ¹³C¹⁸O clumping in organic molecules. Using 32 model molecules, we calculate a total of 143 Δ values (Δ representing the deviation from the stochastic distribution due to equilibrium isotope effects) at temperatures ranging from 300 K to 1000 K. The overall trend follows: ΔDD > Δ¹³CD > Δ¹³C¹⁸O > Δ¹³C¹³C ≈ Δ¹³C¹⁵N, with values decreasing as temperature increases. Through multiple correlation analysis, we demonstrate that the reduced mass of the atoms, bond multiplicity, and hybridization collectively explain 80 % of the differences observed between bond types. Given current analytical uncertainties, the potential for a geothermometer is primarily limited to DD and ¹³CD clumping, while ¹³C¹³C, ¹³C¹⁵N, and ¹³C¹⁸O clumping could be applicable at low temperatures (typically 100 K) or with instrumental precision one order of magnitude higher.

The calculations presented here provide a framework to assess the instrumental precision required for utilizing clumped isotopes in organic molecules as geothermometers. Future improvements in analytical techniques and computational methodologies could further refine these predictions and broaden the applicability of clumped isotope thermometry in organic geochemistry.