Experimental and theoretical analysis of the kinetics of the reaction of atomic bromine with 1, 4-dioxane

The rate coefficient for the reaction of atomic bromine with 1, 4-dioxane was measured from ∼300 to 340 K using the relative rate method. Iso-octane and iso-butane were used as reference compounds, and the experiments were made in a bath of argon containing up to 210 Torr of O₂ at total pressures between 200 and 820 Torr. The rate coefficients were not affected by changes in pressure or O₂ concentration over our range of experimental conditions. The ratios of rate coefficients for the reaction of dioxane relative to the reference compound were put on an absolute basis by using the published absolute rate coefficients for the reference reactions. The variation of the experimentally determined rate coefficients with temperature for the reaction of Br with 1, 4-dioxane can be given by k₁^exp(T) = (1.4 ± 1.0) × 10^-11exp[-23.0 ± 1.8) kJ mol ^-1/(RT)] cm³ molecule^-1 s^-1. We rationalized our experimental results in terms of transition state theory with molecular data from quantum chemical calculations. Molecular geometries and frequencies were obtained from MP2/ aug-cc-pVDZ calculations, and single-point energies of the stationary points were obtained at CCSD(T)/CBS level of theory. The calculations indicate that the 1, 4-dioxane + Br reaction proceeds in an overall endothermic addition- elimination mechanism via a number of intermediates. The rate-determining step is a chair-to-boat conformational change of the Br-dioxane adduct. The calculated rate coefficients, given by k₁^calc(T) = 5.6 × 10^-11exp[-26.6 kJ mol^-1/(RT)] cm³ molecule^-1 s^-1, are in very good agreement with the experimental values. Comparison with results reported for the reactions of Br with other ethers suggests that this multistep mechanism differs significantly from that for abstraction of hydrogen from other ethers by atomic bromine.