¹Shiori Inada, ²Tetsuya Hama, ^1,3Shogo Tachibana
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2025.07.018]
¹Department of Earth and Planetary Science, The University of Tokyo, 7-3-1 Hongo, Tokyo 113-0033, Japan
²Komaba Institute for Science and Department of Basic Science, The University of Tokyo, 3-8-1 Komaba, Tokyo 153-8902, Japan
³UTokyo Organization for Planetary and Space Science, The University of Tokyo, 7-3-1 Hongo, Tokyo 113-0033, Japan
Copyright Elsevier

IIsotopic fractionation resulting from kinetic isotope effects (KIEs) in evaporation is a key to investigating high-temperature evaporation events in the early Solar System. The magnitude of the KIEs is represented by the kinetic isotope fractionation factor , which is predicted as  (: the mass ratio of the isotopic evaporated gas species) to a first approximation based on the Hertz-Knudsen equation. However, the experimentally measured  are often closer to 1 than this prediction to various degrees. In this study, we investigated the reason for this observation based on the transition state theory. To evaluate the classical (high-temperature) limit of , which is given by the isotopic ratio of the imaginary frequencies representing the evaporative motion at the transition state, we constructed a simple model for the vibrational normal mode analysis. In this model, we included the effects of the interaction of the evaporating species with the condensed phase surface, as well as the degrees of freedom of atoms in the condensed phase. The present theory clarified the relationship between the magnitude of the evaporative KIEs and the properties of the potential energy surface: the classical limit of  becomes closer to 1 than  due to the effect of the condensed-phase degrees of freedom when there exists a potential energy barrier, which is related to unstable interaction between the evaporating species and the condensed phase surface. This result is consistent with the previous experimental data and provides general insights into classical KIEs in chemical reactions.