Symmetry induced kinetic isotope effects in the formation of (CO2)+2

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Abstract
Symmetry induced kinetic isotope effects (SIKIE) have been observed mass spectrometrically in the termolecular association reaction of CO+2 and CO2 to produce (CO2)+2, where CO+2 is produced by electron ionization. Distinct and independent kinetic effects are determined for 13C and 18O isotopic substitution making this reaction system the first one for which SIKIE is observed for two different elements. The results can be understood generally in terms of a recently proposed diatom/diatom symmetry correlation scheme extended to treat the present case of linear triatomic molecules. The 18O SIKIE shows a pronounced dependence on the ionizing electron energy decreasing smoothly from a factor of 27 at 25 eV to a factor of 8 at 100 eV. Alternatively, the 13C SIKIE was about a factor of 5 independent of the ionizing electron energy. In addition, the efficiency of CO+2 ions in producing (CO2)+2 increases with increasing energy of the ionizing electrons. These dependencies on ionizing electron energy are interpreted in terms of propensities for e/f parity label state production in electron ionization in qualitative agreement with similar conclusions reached in a recent study of SIKIE in O+4 formation.