The kinetic energy dependence of association reactions. A new thermokinetic method for large systems

The reactions of bare alkali metal ions (M+=Li+, Na+, or K+) with dimethoxyethane (CH3OCH2CH2OCH3, DXE) are studied using guided ion beam tandem mass spectrometry. The bimolecular reaction forms an associative M+(DXE) complex that is long-lived and dissociates back to the reactants. The kinetic energy dependences of the cross sections for formation of the complexes are interpreted with several different models (including rigorous phase space theory) that assume that the complex lifetimes are limited by dissociation over a loose, orbiting transition state. After accounting for the effects of multiple ion-molecule collisions, internal energy of the reactant ions, Doppler broadening, and dissociation lifetimes, the analyses yield 0 K bond energies as the only adjustable parameter. These values are compared with bond energies obtained from previous collision-induced dissociation (CID) studies of the M+(DXE) complexes and found to be self-consistent for all models studied. Association and CID form the same energized M+(DXE) complex in two distinct ways, such that a comparison of these results allows an assessment of the models used to interpret CID thresholds and test the limits of statistical theories such as RRKM and phase space theory. (C) 2003 American Institute of Physics.