Position-specific and clumped isotope equilibria in propane: Ab initio calculations beyond the harmonic and Born-Oppenheimer approximations

Abstract

Position-specific and clumped isotope compositions that reveal intramolecular isotope distributions can offer novel insights into the physical and chemical properties of substances. In particular, the intramolecular isotope effects observed in propane have demonstrated significant potential for constraining the formation and evolution of hydrocarbons. To calibrate measurements and interpret observations, a comprehensive understanding of equilibrium isotope fractionation within propane is required. However, previous studies yielded inconsistent calculations of position-specific isotope equilibria, and lacked any prediction of clumped isotope equilibria in the methyl group of propane. Here, we employ quantum chemical calculations beyond the harmonic approximation and the Born-Oppenheimer approximation to study the intramolecular isotope equilibria in propane. Benchmark coupled cluster calculations (CCSD(T)) were used to produce reliable frequencies of propane isotopologues. By integrating the CCSD(T) outcomes with effects beyond the harmonic and Born-Oppenheimer approximations, we present high-accuracy equilibrium carbon and hydrogen isotope fractionation between the methylene and methyl sites, and report for the first time the equilibrium Δ13CH2D and Δ12CHD2 signatures of the methyl group in propane. Our results of position-specific isotope equilibria can serve as a reference frame for calibrating both theoretical calculations and experimental measurements. The equilibrium Δ13CH2D and Δ12CHD2 values provide a theoretical framework for further studies of 13CD and DD clumping in methyl groups of alkanes, and are valuable for tracing the sources and sinks of methyl groups as well as processes related to methylation.