Kinetics and Mechanism of the Reaction of F Atoms with CH₃Br

Abstract

The reaction of F atoms with CH 3 Br at 296 K was studied using a pulse radiolysis/transient UV absorption spectroscopy absolute technique and a FTIR relative rate technique. The rate constant for this reaction was determined to be (4.46 ( 0.22) × 10 -11 . The reaction proceeds via two channels, 69 ( 5%, via hydrogen abstraction giving CH 2 Br radicals and HF, and 31 ( 5%, to give the adduct CH 3 Br--F. In the FTIR system the observed rate constant was 69 ( 8% of that measured using the pulse radiolysis system because the CH 3 Br--F adduct falls apart to re-form the reactants. The CH 3 Br--F adduct reacts with NO, with a rate constant of (2.25 ( 0.14) × 10 -11 cm 3 molecule -1 s -1 , giving FNO as a product. There was no discernible reaction of the CH 3 Br--F adduct with O 2 and an upper limit of 6 × 10 -15 cm 3 molecule -1 s -1 was derived for this reaction. The CH 3 Br--F adduct absorbs strongly at 260-340 nm. The absorption cross section at 280 nm of the CH 3 Br--F adduct was (2.06 ( 0.31) × 10 -17 cm 2 molecule -1 . A lower limit for the equilibrium constant was [CH 3 Br--F]/([CH 3 Br][F]) > 5 × 10 -16 cm 3 molecule -1 at 296 K. A lower limit of 12 kcal mol -1 is estimated for the binding energy of the F atom in the CH 3 Br--F adduct. The UV absorption spectrum of the CH 2 BrO 2 radical was determined; at 250 nm σ ) (3.4 ( 0.9) × 10 -18 cm 2 molecule -1 .

Oxidation of dimethyl ether: Absolute rate constants for the self reaction of CH3OCH2 radicals, the reaction of CH3OCH2 radicals with O2, and the thermal decomposition of CH3OCH2 radicals

“…The values of k 1 obtained by Masaki et al [15] are independent of total pressure in the pressure range 0.77 -7.7 mbar. This is, however, in line with the results reported by Sehested et al [7] who found that k 1 is independent of total pressure in the pressure range 0.77 -7.7 mbar within 25%. This was explained by the following mechanism for reaction (1):…”

“…Sehested et al [7] obtained values for k prod,0 , and k RO2,0 , and k RO2,ϱ relative to k CH3OCH2ϩC12 of cm 3 and Sehested et al [7] determined using the absolute rate constants obtained in this work and those obtained by Masaki et al [15]. Hence, only the rate constant ratios k RO2,ϱ /k prod,0 and k RO2,ϱ /k prod,0 obtained in this work and the work of Sehested et al [7] can be compared directly.…”

“…The atmospheric chemistry of DME has been studied by Japar et al [3], Jenkin et al [4], Wallington et al [5], Langer et al [6], and Sehested et al [7]. Jenkin et al [4] and Sehested et al [7] reported high formaldehyde yields following chlorine initiated oxidation of dimethyl ether in a N 2 /O 2 diluent at total pressures below 300 torr.…”

“…Jenkin et al [4] and Sehested et al [7] reported high formaldehyde yields following chlorine initiated oxidation of dimethyl ether in a N 2 /O 2 diluent at total pressures below 300 torr. It was shown that formaldehyde is formed from the reaction of CH 3 OCH 2 radicals with O 2 via the following mechanism [4,7]: [3 -5].…”

Matrix-isolated infrared absorption spectrum of CH2BrOO radical

“…For instance, bromomethylperoxy radical (CH 2 BrOO) can be generated via reaction CH 2 Br + O 2 in the atmosphere (especially in the marine boundary layer), while the CH 2 Br radicals are formed via abstraction of a hydrogen atom from CH 3 Br by OH or Cl [6,7]. The UV absorption spectrum of gaseous CH 2 BrOO has been studied, revealing a broad and structureless feature at 240 nm [2,4,8]. The rate coefficient [4] of its self-reaction has been determined to be (1.1 ± 0.4) Â 10 À12 cm 3 molecule À1 s À1 at 298 K. Recently, the IR spectrum of CH 2 BrOO has been studied by Huang and Lee [9].…”