Path-integral methods for clumped and position-specific isotope studies

Abstract

Recently developed methods for exptl. anal. of isotopic enrichment can detect the enhanced thermodn. stability of a specific placement or clumping of rare isotopes. Such measurements, which depend principally on homogeneous fractionation phenomena, provide powerful tools to deduce formation temps., to infer the origin of samples, or to place addnl. isotopic constraints on systems of interest. In such cases, theor. predictions regarding equil. enrichment are essential for establishing an abs. ref. frame for standardizing isotope ratio measurements and assessing the presence of potential kinetic factors. For decades, the primary theor. framework for characterizing equil. isotope effects has been the Urey model. In this approach, however, effects assocd. with vibrational anharmonicity and ro-vibrational coupling are ignored. This is potentially problematic given the nature of this subtle fractionation behavior and the increasing sensitivity of anal. instrumentation. We utilize path-integral methods with high-quality potential energy surfaces to rigorously calc. equil. isotope effects to the same level of precision as the best anal. instrumentation currently available. Representative calcns. that are relevant to 'clumped' and position-specific stable isotope anal. of mols. like CO2, N2O, methane, and ethane are performed to illustrate the utility of path-integral methods for geochem. applications, to identify errors in existing theor. predictions, and to quantify the potential impact on exptl. detns.