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.
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.
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