Kinetic studies of the reactions of ground state atomic carbon, [formula omitted], with halogenated methanes investigated by time-resolved atomic resonance absorption spectroscopy in the vacuum ultra-violet

Abstract

A kinetic investigation is presented of the reactions of ground state atomic carbon, C(2p 2( 3P J)) , with the molecules methyl chloride, methyl bromide, methyl iodide, methyl fluoride, dichloromethane, chloroform and carbon tetrachloride. Atomic carbon was generated by the repetitive pulsed irradiation (λ > ca. 160 nm) of C 3O 2 in the presence of excess helium buffer gas and the added reactant gases in a slow flow system, kinetically equivalent to a static system. C(2 3 P J ) was then monitored photoelectrically by time-resolved atomic resonance absorption in the vacuum ultraviolet (λ = 166 nm, 3 3 P J ← 2 3 P J ) with direct computer interfacing for data capture and analysis. The following absolute second-order rate constants for the reactions of C(2 3 P J ) with the above collision partners are reported (errors 2σ): Reactant k R/ cm 3 molecule −1 s −1 (300 K) CH 3Cl (4.5 ± 0.3) × 10 −12 CH 3Br (5.2 ± 0.3) × 10 −11 CH 3I (5.3 ± 0.3) × 10 −11 CH 3F (1.3 ± 0.1) × 10 −12 CH 2Cl 2 (5.9 ± 0.3) × 10 −12 CHCl 3 (8.9 ± 0.5) × 10 −12 CCl 4 (1.5 ± 0.1) × 10 −11 These yield, to the best of our knowledge, the first reported body of absolute rate data for reactions of ground state carbon with these reactants. Although a large body of absolute rate data for atomic silicon in its Si(3P 2( 3P J)) ground state has now been determined directly in recent years by time-resolved atomic resonance absorption spectroscopy in the ultraviolet itself with a wide range of collision partners, there are no analogous data for these particular reactants with which the data for C(2 3 P J ) may be compared. Estimates of the small energy barriers consistent with the rate data for C(2 3 P J ) + CH 3 Cl, Br, I coupled with the bond dissociation energies of these polyatomic reactants in particular confirms the values of the bond dissociation energies for CCl, Br, I derived from mechanistic interpretation on C 2 Swann band emission from alkali metal-organic halide diffusion flames.