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

摘要

Guided ion beam tandem mass spectrometry techniques are used to measure the reaction cross sections of the collisionally activated process I~-+CH_3Y-> products,where Y=Cl and Br.The Cl~ and Br~ products are observed at the lowest collision energies.A back-side attack S_N2 reaction is responsible for the initial rise from the threshold.At higher collision energies,the IC1"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,DELTAH_0.The reaction mechanisms are explored with the aid of CCSD(T)/LanL2DZ and CCSD(T)/SDD molecular orbital calculations and phase space theory.