The computational study of the “inversion substitution” reactions CX 3Br + O 2 → CX 3O 2 + Br (X = H, F)

Abstract

The kinetic parameters for the “inversion substitution” reactions of Br-containing fire suppressants CH 3Br and CF 3Br with molecular oxygen CX 3Br + O 2 → CX 3O 2 + Br (X = H, F) have been obtained on the basis of computational study carried out by quantum chemistry methods and transition state theory (TST). The DFT/B3LYP approach has been used to obtain molecular structures and vibrational frequencies and CCSD(T) method with additive corrections at MP2 and SODFT levels have been applied in order to calculate reaction enthalpies and activation barriers. Activation barrier for the substitution reaction CH 3Br + O 2 → CH 3O 2 + Br was found to be (53.3 kcal/mol) close to the endoergicity (51.2 kcal/mol) of bimolecular abstraction CH 3Br + O 2 → CH 2Br + HO 2 which means that this “inversion substitution” should be considered as a competing process in the oxidation initiation of bromine containing substances. The results of these calculations together with the earlier results have allowed us to build the correlation of Evans–Polanyi type between the activation barrier and reaction enthalpy for “inversion substitution” reactions CH 3X + O 2 → CH 3O 2 + X. For bromotrifluoromethane CF 3Br (Halon-1301) the barrier of “inversion substitution” was found to be of 86 kcal/mol that is higher by about 15 kcal/mol as compared with unimolecular decay CF 3Br → CF 3 + Br barrier. This makes negligible the role of bimolecular substitution reaction for kinetics of CF 3Br in oxygen environment. The TST calculations of the rate constants and their temperature dependence for the direct and reverse “inversion substitution” reactions have been carried out within the temperature interval of 273–2000 K.