Reaction dynamics of ion-molecule systems: Crossed beam studies of hydrogen atom transfer and isotope exchange

Gas phase ion-molecule reaction dynamics have been examined through the method of crossed molecular beams. Extensive research on the reaction of O -- with molecular hydrogen and deuterium has been performed over the collision energy range 0.20--1.20 eV. In the first part of that study, an anomalous isotope effect on the product energy partitioning was observed. The fraction of the total available energy deposited into product vibration was significantly larger for reaction with deuterium than with hydrogen, especially at lower collision energies. These findings are discussed in terms of competition between two reaction mechanisms producing atom transfer: an insertion-migration mechanism yielding moderately excited products and a direct mechanism producing more highly excited products. The insertion process leads to the isotope effect, which is explained in terms of Franck-Condon factors. In the second part of the study, the effects of reagent rotational energy on the reaction dynamics were studied. The rotational energy of deuterium was reduced by cooling to liquid nitrogen temperature. At low collision energies, a significant increase in the relative amount of backward scattering in the product angular distributions was found. The angular distributions reflect the specific geometry required for passage through the critical saddle point region of the potential surface where atom transfer and electron detachment compete.; The dynamics of the OH--+D2 system over the collision energy range 0.27 to 0.67 eV have been investigated. The isotope exchange reaction shows insensitivity to collision energy. Although an intermediate complex is formed, the OD-- products are primarily backward scattered, The angular distributions are interpreted in terms of the osculating model. Nonreactive scattering has proven useful in the interpretation of the product energy partitioning. Structure in the OH -- recoil energy distributions indicates the deposit of specific amounts of energy into rotational modes. This rotational excitation develops from vibrational motion in the intermediate complex orthogonal to the reaction coordinate. The rotational energy partitioning argues that the reaction dynamics could be governed by passage of the intermediate complex through quantized transition state thresholds.