19436
J. Phys. Chem. 1996, 100, 19436-19440
Rate Coefficients for Dissociative Electron Attachment by Halomethane Compounds between 300 and 800 K Steven J. Burns, Jeanne M. Matthews, and David L. McFadden* Eugene F. Merkert Chemistry Center, Department of Chemistry, Boston College, Chestnut Hill, Massachusetts 02167 ReceiVed: August 16, 1996; In Final Form: October 2, 1996X
The temperature dependence of the rate coefficients for thermal energy dissociative electron attachment reactions has been measured for nine halomethane compounds. Rate coefficients for CCl4, CFCl3, CF2Cl2, CF3Cl, CHCl3, CH2Cl2, CHFCl2, CH3I, and CH2Br2 were determined between 300 and 800 K from relative rate measurements by using SF6 as a reference compound. A flowing afterglow-electron cyclotron resonance technique was employed. Mass spectral analysis of product negative ions confirmed a dissociative attachment mechanism. Semiempirical quantum mechanical calculations of the geometry change associated with negative ion formation were carried out.
Introduction The study of the temperature dependence of dissociative electron attachment in the gas phase by thermal energy electrons began in the 1960s with a study that included several halomethane compounds.1,2 Since then there have been more than 30 kinetics studies of halomethane reactions in which thermal energy rate coefficients have been determined, including five investigations of the temperature dependence of the rate coefficients (see references listed in Table 1 and also ref 3). Electron swarm and electron beam studies have provided extensive information on both the temperature dependence and the energy dependence of dissociative attachment reactions.4-28 A low-energy (
