Mass-independent isotopic fractionation in nonadiabatic molecular collisions

Abstract

Symmetry and parity constraints associated with nonadiabatic collisions are shown to result in selection rules that can produce mass-independent isotopic fractionation between molecular electronic states coupled by a nonadiabatic process. Nonadiabatic transitions from Π or Δ molecular electronic states to ∑ electronic states can result in isotopic fractionation for atoms occurring in equivalent positions in the molecule if the common isotope of those atoms is a spin-zero nucleus. The Π or Δ state becomes depleted of the rare isotopes of those atoms while the ∑ state is enriched in the rare isotopes. Chemical processes that distinguish between the Π or Δ and ∑ states can convert this isotopic fractionation between electronic states of the same chemical species to a fractionation between different chemical species. Such nonadiabatic-collision-induced isotopic fractionation might explain observations of mass-independent isotopic fractionation in electrical discharges, as well as in meteoritic samples. Nonadiabatic-collision-induced isotopic fractionation also may provide an explanation for observations of isotopic enrichments in the earth’s atmosphere.