Gas-Phase Reactions of the Iodide Ion with Chloromethane and Bromomethane: Competition between Nucleophilic Displacement and Halogen Abstraction

Guided ion beam tandem mass spectrometry techniques are used to measure the reaction cross sections of the collisionally activated process I - + CH 3 Y → products, where Y = Cl and Br. The Cl - and Br - products are observed at the lowest collision energies. A back-side attack S N 2 reaction is responsible for the initial rise from the threshold. At higher collision energies, the ICl - and IBr - ions are observed and signify competition from a front-side-attack, halogen-abstraction reaction. All the reactions are endoergic and exhibit excess threshold energies, E 0, when compared with established reaction endothermicities from the literature, Δ H 0 . The reaction mechanisms are explored with the aid of CCSD(T)/LanL2DZ and CCSD(T)/SDD molecular orbital calculations and phase space theory.