Low Pressure Gas-phase Reactions of the Atomic Oxygen Radical Anion with Halomethanes Studied Using Fourier Transform Ion Cyclotron Resonance

Abstract

The gas-phase reactions of the O−• radical anion with the halomethanes CH3X, CH2X2, CHX3, CX4, CF3X, CF2X2, CFX3 (X = Br and Cl) and CXClBr2 (X = Cl and F) have been examined at a low pressure (10−5–10−4 Pa) with use of Fourier transform ion cyclotron resonance (FTICR) mass spectrometry. The overall reactions proceed essentially with the collision rate and lead to a variety of product ions dependent on the number and nature of the halogen atoms present in the substrate. For CH3CI and CH3Br, the dominant pathways are H• abstraction, H2+• abstraction and nucleophilic substitution. With CH2CI2, proton transfer is an additional major pathway and in the reaction with CH2Br2 attack on a halogen atom with formation of BrO− ions also occurs. In the reactions with CHCl3 and CHBr3, proton transfer competes with attack on a halogen atom, whereas initial attack on a halogen atom dominates in the reactions of O−• with the CCl4 and CBr4 methanes. Attack on the carbon atom is the main process if CF3Cl is the substrate and results in F−, Cl− as well as FCl−• ions. For CF3Br, however, attack on the bromine atom with formation of BrO− ions dominates over the reaction at the carbon atom. In the reactions with CF2Cl2, CFCl3, CF2Br2, as well as the CCl2Br2 and CFClBr2 methanes, most product ions are formed by competing attack on a chlorine and bromine atom. For some of the halomethanes, the present findings are compared with reported results obtained with use of the high pressure (≡70 PA) flowing afterglow and selected ion flow tube methods. © 1997 John Wiley & Sons, Ltd.