Intramolecular isotope effects in the reactions of CF32+ and CO22+ with HD

Intramolecular isotope effects in the bond forming reactions following collisions of both CO22+ and CF32+ with HD have been investigated experimentally. For the CO22++HD system the bond-forming pathway forming XCO+ (X=H, D) exhibits a strong intramolecular isotope effect favoring the formation of DCO+ at low collision energies. For the CF32++HD system the bond-forming pathway forming XCF2+ also exhibits a strong intramolecular isotope effect favoring the formation of DCF2+ at low collision energies. However, in the CF32++HD system a weak, and previously unobserved, channel, forming XF+ exhibits no intramolecular isotope effect over the collision energy regime (0.2–0.5 eV) investigated. The absence of an intramolecular isotope effect in the formation of XF+ casts doubt on the previous explanation of such isotope effects as resulting from orientation effects in the approach of the dication to the HD molecule. Using a recently proposed mechanism for the reaction of CO22+ with H2, an analysis of the statistical and zero-point factors affecting the competition between the bond-forming channels is presented. This analysis shows that such factors can readily explain the intramolecular isotope effects observed in these reactive systems.