Chemistry as a function of solvation number. Solvated-ion reations in the gas phase and comparison with solution

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Abstract
Reactions of the hydroxide ion OH–(H2O)0,1,2,3, solvated with 0–3 water molecules, have been studied in the gas phase using flow-tube and ion-beam techniques. The reactions studied included proton transfer, nucleophilic displacement and isotope exchange. The experiments measure, as a function of the reactant solvation number, the dependence of rate constant on temperature or of cross-section on energy, for each of the possible solvation numbers of the products. The experiments reveal the pathway of the solvent molecules in the reaction and their kinetic role in facilitating or quenching reaction. Proton transfer is usually facile, unaffected by solvent molecules, giving solvated products: solvent molecules quench nucleophilic displacement, giving unsolvated products. For solvent molecules to transfer from reactants to product, they must track the charge as it moves in the reaction intermediate. This is governed by the structure of the transition state, possible for proton transfer but not for nucleophilic displacement. A complete description of the isotope exchange reaction OH–+ D2 OD–+ HD is given throughout the full range of solvation. The gas-phase studies identify the intermediate in solution and predict correctly the activation energy in solution. Throughout we consider if solvated-ion reactions ‘bridge the gap’ between studies without solvation in the gas phase and studies with complete solvation in solution.

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