In this project in the Physical Chemistry Program of the Chemistry Division, Dr. Veronica Bierbaum of the University of Colorado will study the kinetics, mechanisms, and thermochemistry of gas phase ion-molecule reactions using tandem flowing afterglow-selected ion flow tube instruments. Topics for study include proton transfer in simple systems, isotope exchange in associative detachment reactions, solvation and kinetic isotope effects in nucleophilic substitution reactions of hydrated ions, rate coefficient and product distribution measurments of reactions of atmospheric interest, and the determination of thermochemical data for halogen- and sulfur-containing compounds. %%% The study of gas phase ion chemistry affords the opportunity to obtain a fundamental understanding of species and processes which are naturally present primarily in solution, in solids, or in the upper atmosphere. By studying reactions between ionic and neutral atoms or molecules without surrounding solvent or other neighboring species, the structure, reactivity, and thermodynamic properties of the ionic atoms or molecules can be determined. Applications span a broad variety of fields, from medical and analytical to environmental.