A theoretical kinetic study on the reaction of atomic bromine with toluene

Abstract

The reaction of Br atoms with toluene was investigated by employing various quantum chemical methods and statistical rate theory calculations. Various composite methods such as CBS-QB3, G3, and G4 were used to obtain the energy profiles of the Br + toluene reaction. Further single-point calculations of the stationary points were performed at the CCSD(T)/cc-pV(D,T)Z level of theory using B3LYP/cc-pVTZ and MP2/aug-cc-pVDZ optimized geometries. Our calculations revealed several reaction pathways in the potential energy surface of the Br + toluene reaction. However, the reaction pathway that abstracts hydrogen atoms from the methyl site of toluene was found to be energetically the most favorable. This reaction pathway appears to proceed via a complex forming mechanism, similar to that seen in the reactions of cyclic ethers with Br atoms. Our calculations reveal that the reaction of a Br atom with toluene proceeds exclusively via intermediate complexes in an overall endothermic addition-elimination mechanism. Based on the ab initio results, the standard enthalpies of formation of the product radicals and the rate coefficients for the relevant reaction pathways are computed. The calculated values of the enthalpy of formation are found to match excellently with the available literature data. Lowering the barrier height of hydrogen abstraction reaction at the methyl site by less than 4 kJ/mol, the calculated rate coefficients, kov(T) = 1.36 × 10–23 T3.687 exp(−4.57 K/T) cm3 molecule–1 s–1, reproduced the experimental data excellently from 200 to 500 K.