Theoretical calculation of position-specific carbon and hydrogen isotope equilibriums in butane isomers

Abstract

Position-specific isotope analysis has shown its potential to reveal information regarding formation, migration, and conversion procesess of hydrocarbons. The intramolecular isotope compositions of butane are promising to serve as a new thermometer and tracer. Therefore, position-specific isotope signatures in butane at equilibrium are needed for calibrating experimental measurements, establishing new geothermometers, and recognizing kinetic isotope effects. Here we conduct quantum chemistry modeling with corrections beyond the harmonic approximation and the Born-Oppenheimer approximation to obtain accurate intramolecular and intermolecular carbon and hydrogen isotope fractionation factors for butane isomers at equilibrium. Temperature dependences of these equilibrium isotope effects are presented for the range from 0 °C to 726.85 °C. The contribution of higher-order energy terms to 1000lnα values beyond the Bigeleisen-Mayer equation is found to be comparable to the magnitude of current experimental precisions. In addition to the significance of anharmonicity, the contribution of hindered internal rotation and diagonal Born-Oppenheimer correction is found to be important for accurate predictions of position-specific hydrogen isotope equilibriums. The abundance ratio of n-butane to i-butane at equilibriums is also calculated at various temperatures. Our results provide fundamental understanding of equilibrium properties for studying position-specific isotope effects in butane.