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.

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An experimental and computational study has been performed to investigate the bond-forming reactivity between Ar(2+) and NH(3). Experimentally, we detect two previously unobserved bond-forming reactions between Ar(2+) and NH(3) forming ArN(+) and ArNH(+). This is the first experimental observation of a triatomic product ion (ArNH(+)) following a chemical reaction of a rare gas dication with a neutral. The intensity of ArNH(+) was found to decrease with increasing collision energy, with a corresponding increase in the intensity of ArN(+), indicating that ArN(+) is formed by the dissociation of ArNH(+). Key features on the potential energy surface for the reaction were calculated quantum chemically using CASSCF and MRCI methods. The calculated reaction mechanism, which takes place on a singlet surface, involves the initial formation of an Ar-N bond to give Ar-NH(3)(2+). This complexation is followed by proton loss via a transition state, and then loss of the two remaining hydrogen atoms in two subsequent activationless steps to give the products (3)ArN(+) + H(+) + 2H. This calculated pathway supports the sequential formation of ArN(+) from ArNH(+), as suggested by the experimental data. The calculations also indicate that no bond-forming pathway exists on the ground triplet surface for this system.