KINETICS AND MECHANISM OF THE REACTION OF F ATOMS WITH CH3BR

The reaction of F atoms with CH3Br 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 w as determined to be (4.46 +/- 0.22) x 10(-11). The reaction proceeds v ia two channels, 69 +/- 5%, via hydrogen abstraction giving CH2Br radi cals and HF, and 31 +/- 5%, to give the adduct CH3Br--F. In the FTIR s ystem the observed rate constant was 69 +/- 8% of that measured using the pulse radiolysis system because the CH3Br--F adduct falls apart to re-form the reactants. The CH3Br--F adduct reacts with NO, with a rat e constant of (2.25 +/- 0.14) x 10(-11) cm(3) molecule(-1) s(-1), givi ng FNO as a product. There was no discernible reaction of the CH3Br--F adduct with O-2 and an upper limit of 6 x 10(-15) cm(3) molecule(-1) s(-1) was derived for this reaction. The CH3Br--F adduct absorbs stron gly at 260-340 nm. The absorption cross section at 280 nm of the CH3Br --F adduct was (2.06 +/- 0.31) x 10(-17) cm(2) molecule(-1). A lower l imit for the equilibrium constant was [CH3Br--F]/([CH3Br][F]) > 5 x 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 CH3Br--F adduct . The UV absorption spectrum of the CH2BrO2 radical was determined; at 250 nm sigma = (3.4 +/- 0.9) x 10(-18) cm(2) molecule(-1).