A theoretical analysis of the kinetics of the reaction of atomic bromine with tetrahydrofuran

Abstract

The kinetic behaviour for the reaction of atomic bromine with tetrahydrofuran has been analysed using the information from quantum chemical calculations. Structures and energy profiles were first obtained using density functional theory (DFT) employing the Dunning’s basis sets of triple-zeta quality, and then for an accurate energetic description, single-point calculations were carried out at the coupled-cluster with single and double excitations (CCSD) and the fourth-order Møller–Plesset (MP4(SDQ)) levels of theory. The rate coefficients and the equilibrium constants for the potential reaction channels were obtained from the statistical rate theories and statistical thermodynamics, respectively, using the results of quantum chemical calculations; and the results were compared with our recently published experimental data. In terms of reaction mechanism, this reaction was found to be analogous to the reactions of the Br atom with 1,4-dioxane and with methanol, where the reaction proceeds via an addition–elimination mechanism. The dominant reaction channel involved coordination of the approaching Br atom to one of the hydrogen atoms adjacent to the ether oxygen atom, i.e., β-hydrogen abstraction is uncompetitive. Although the complexes formed by direct coordination of the Br atom to the ether oxygen atom appeared in the reaction mechanism, we were not able to link them specifically to any reaction. The density functional theory predicted an activation energy and enthalpy of reaction that were much smaller than the experimental values, which led to an overestimation of the theoretical rate coefficients. The source of this discrepancy could be attributed to the overbinding of the transition states and of the tetrahydrofuranyl radical by DFT. Single-point calculations at the DFT structures using the CCSD and MP4(SDQ) methods yielded an accurate energetic description of the reaction of tetrahydrofuran with bromine, resulting in rate coefficients that showed excellent agreement with the experimental values.