Flame Inhibition Chemistry: Rate Coefficients of the Reactions of HBr with CH₃ and OH Radicals at High Temperatures Determined by Quasiclassical Trajectory Calculations

Abstract

Reactions of HBr with radicals are involved in atmospheric chemistry and in the mechanism of operation of bromine-containing flame retardants. The rate coefficients for two such reactions, HBr + OH and HBr + CH 3 , are available from earlier experiments at near or below room temperature, relevant for atmospheric chemistry, and in this domain, the activation energy for both has been found to be negative. However, no experimental data are available at combustion temperatures. In this work, to provide reliable data needed for modeling the action of brominated flame suppressants, we used the quasiclassical trajectory (QCT) method in combination with high-level ab initio potential energy surfaces to evaluate the rate coefficients of the two title reactions at combustion temperatures. The QCT calculations have been validated by reproducing the experimental rate coefficients at room temperature. At temperatures between 600 and 3200 K, the QCT rate coefficients display positive activation energies. We recommend the following extended Arrhenius expressions to describe the temperature dependence of the thermal rate coefficients: k 6 = (9.86 ± 2.38) × 10 −16 T (1.23±0.03) exp[(5.93 ± 0.33) kJ mol −1 /RT] cm 3 molecule −1 s −1 for the HBr + OH → H 2 O + Br reaction, and k −2 = (4.06 ± 2.72) × 10 −18 T (1.83±0.08) exp[(7.53 ± 0.18) kJ mol −1 /RT] cm 3 molecule −1 s −1 for the HBr + CH 3 → CH 4 + Br reaction. The latter is in very good agreement with the formula proposed by Burgess et al. [

Rate Constant of the Reaction of OH Radicals with HBr over the Temperature Range 235–960 K

“…This expression reproduces quite well the temperature dependence of k 1 measured in the current study between 600 and 1000 K, with the absolute values of k 1 by 20% higher compared with the experimental data. Similarly, the QCT calculations by Góger et al 15 predict a small positive temperature dependence of k 1 at high temperatures, but in absolute value, they are by a factor of  1.8 lower at T> 600 K than the current experimental results. Experimental data for k 1 obtained at ultralow temperatures using supersonic Laval nozzle flow reactors with monitoring of OH by laser induced fluorescence method [6][7][8][9][10] were analyzed by Mullen and Smith 6 who recommended k 1 = (1.06±0.02)×10 -11 (T/298) (-0.9±0.11) cm 3 molecule -1 s -1 in the temperature range 23-360 K. This expression obtained from the fit to the experimental data at low temperatures and that from the present work describing behavior of k 1 at high temperatures give identical (within 1%) results for k 1 at temperatures 240-270 K.…”

Reaction Rates Control High-Temperature Chemistry of Volcanic Gases in Air

“…Nevertheless, offline calculations of the lifetime of volcanic HCl, HBr emissions can be undertaken to estimate the potential for halogen radical formation, shown here for the Eyjafjallajökull simulations. Ravishankara et al (1985) as k = 4.5 × 10 −17 × T Góger et al (2018) as k = (9.86 ± 2.38) × 10 −16 × T (1.23 ± 0.03) × exp[(5.93 ± 0.33) kJ mol −1 /RT] cm 3 molecule −1 s −1 (valid for 600-3200 K).…”

Polyatomic radiative association by quasiclassical trajectory calculations: Cross section for the formation of HCN molecules in H + CN collisions

“…efficient solution regardless the size of reactants. Despite the lacking quantum effects, the trajectory based method with a good quality potential energy surface can provide accurate rate constants, spectral properties and also reliable atomic-scale mechanism of molecular collisions [46][47][48][49][50][51][52][53] .…”