COLLISIONAL ENERGY-TRANSFER IN BIMOLECULAR ION-MOLECULE DYNAMICS M-2 D2 OR HD)-](MH++H MD++D MH++D OR MD++H)(+(H)

Guided ion beam kinetic energy thresholds in the ion-molecule reaction s M++H-2-->MH++H, where M+=B+, Al+, and Ga+ exceed by 0.4- approximate ly 5 eV the thermodynamic energy requirements or theoretically compute d barrier heights of these reactions. In addition, the formation of MD + occurs at a significantly lower threshold than MH+ when M+ reacts wi th HD. Moreover, the measured reaction cross sections for production o f MH+ product ions are very small (10-(17)10(20) cm2) . These facts su ggest that a ''dynamical bottleneck'' may be operative in these reacti ons. In this work, the eigenvalues of the mass-weighted Hessian matrix , which provide local normal-mode frequencies, are used to identify lo cations on the ground-state MH2+ potential energy surfaces where colli sional-to-internal energy transfer can readily take place. In particul ar, the potential energies at geometries where eigenvalues correspondi ng to interfragment and to internal motions undergo avoided crossings are related to the kinetic energies of apparent reaction thresholds. T his near-resonance energy transfer model, applied to M++HD reactions, displays the experimentally observed preference to form MD+ at lower c ollision energies than MH+ as well as the fact that reaction threshold s may greatly exceed thermodynamic energy requirements. This model exp lains the small reaction cross sections in terms of high energy conten t and subsequent dissociation of nascent MH+ (or MD+) ions. Although t he mass-weighted Hessian matrix is used as a tool in this analysis, th e model put forth here is not equivalent to a reaction-path Hamiltonia n dynamics approach.