EPR kinetic study of the reactions of F and Br atoms with H2CO

Experimental and theoretical study of the F, Cl and Br reactions with formaldehyde and acetaldehyde

“…6 Kinetic data for the reactions X ] HCHO ] "" products ÏÏ have previously been obtained by several methods : X \ F,7 Cl8h14 and Br. 7,10,15 The kinetic isotope e †ect in the Cl reaction with HCHO/DCDO, was reported as k Cl`HCHO /k Cl`DCDO , 1.3. 8 We have recently reported an unusually large kinetic isotope e †ect in the bromineÈformaldehyde reaction, with a ratio of around 7.16 There are also k Br`HCHO /k Br`DCDO several reports on the kinetics of the reactions X \ F,17h19 Cl,12,20h24 and X ] CH 3 CHO ] "" products ÏÏ : Br.25h31…”

Temperature-dependent rate coefficient measurements for the reaction of bromine atoms with a series of aldehydes

“…We find a rate coefficient ratio that is essentially independent of temperature over the range studied, 250 -298 K, while the IUPAC recommendations [18] predict an increasing value for k 2 /k 1 with decreasing temperature. For example, the recommended ratio is 4.4 at 250 K, and 5.6 at 220 K, compared with our measured value of 3.6 at 250 K. Thus, it is apparent that a small systematic problem exists with previous data on reaction (1) and/or (2). At this point, the source of this discrepancy is not obvious.…”

“…Absolute measurements of the rate coefficient for reaction (1) have been obtained in three different studies [1 -3]. While the earliest study, a room-temperature discharge flow/EPR measurement [1] is now believed to be in error due to unwanted losses of Br, the two most recent studies [2,3] (one using a discharge flow/mass spectroscopic technique [2] and one using flash photolysis /resonance fluorescence [3]) are in general agreement. Fits of the combined data set, which encompasses the range 223 -480 K, yield k 1 ϭ 1.7 ϫ 10 Ϫ11 exp(Ϫ800/T) cm 3 molecule Ϫ1 s Ϫ1 , and this expression has been adopted by evaluation panels [17,18] with uncertainties of 30 -40% at 298 K and 60% at 225 K. However, the possibility of the existence of some curvature in the Arrhenius plot has been suggested [3] such that the lower temperature values could be somewhat higher than those currently recommended.…”

“…: HCO ϩ HBr (1) The reactions of Br with the smaller aldehydes, reactions (1 -4), could also be important bromine atom sinks in the marine boundary layer [9 -12], particularly during polar surface ozone depletion events [10 -12].…”

“…Despite their atmospheric significance, surprisingly few studies of the rate coefficients for the reactions of Br with aldehydes have been conducted [1 -7]. Absolute measurements of the rate coefficient for reaction (1) have been obtained in three different studies [1 -3]. While the earliest study, a room-temperature discharge flow/EPR measurement [1] is now believed to be in error due to unwanted losses of Br, the two most recent studies [2,3] (one using a discharge flow/mass spectroscopic technique [2] and one using flash photolysis /resonance fluorescence [3]) are in general agreement.…”