Symmetry induced kinetic isotope effects in the formation of Ar·CO₂⁺

Symmetry induced kinetic isotope effects (SIKIE) have been observed mass spectrometrically in the termolecular association reaction of Ar and CO₂⁺ to produce Ar-CO₂⁺ making this the first example of SIKIE in a system where the reactants have no atoms in common. The ¹⁸O SIKIE shows a pronounced dependence on whether the CO₂⁺ are produced directly by electron ionization or indirectly by charge transfer from Ar⁺. The results can be understood generally in terms of reaction restrictions based on a symmetry correlation scheme recently developed by one of the authors. This scheme indicates that CO₂⁺ ions can have very different clustering reactivity with Ar depending on whether the ion is in an e (restricted) or an / (allowed) parity label state. Kinetic modeling of the ¹⁸O SIKIE indicates that CO₂⁺(e)/CO₂⁺(f)≈60 for CO₂⁺ produced by 21 eV electron ionization.