Theoretical Study of the Reaction Kinetics of Atomic Bromine with Tetrahydropyran

A detailed theoretical analysis of the reaction of atomic bromine with tetrahydropyran (THP, C₅H₁₀O) was performed using several ab initio methods and statistical rate theory calculations. Initial geometries of all species involved in the potential energy surface of the title reaction were obtained at the B3LYP/cc-pVTZ level of theory. These molecular geometries were reoptimized using three different meta-generalized gradient approximation (meta-GGA) functionals. Single-point energies of the stationary points were obtained by employing the coupled-cluster with single and double excitations (CCSD) and fourth-order Møller–Plesset (MP4 SDQ) levels of theory. The computed CCSD and MP4­(SDQ) energies for optimized structures at various DFT functionals were found to be consistent within 2 kJ mol^–1. For a more accurate energetic description, single-point calculations at the CCSD­(T)/CBS level of theory were performed for the minimum structures and transition states optimized at the B3LYP/cc-pVTZ level of theory. Similar to other ether + Br reactions, it was found that the tetrahydropyran + Br reaction proceeds in an overall endothermic addition–elimination mechanism via a number of intermediates. However, the reactivity of various ethers with atomic bromine was found to vary substantially. In contrast with the 1,4-dioxane + Br reaction, the chair form of the addition complex (<i>c</i>-C₅H₁₀O–Br) for THP + Br does not need to undergo ring inversion to form a boat conformer (<i>b</i>-C₄H₈O₂–Br) before the intramolecular H-shift can occur to eventually release HBr. Instead, a direct, yet more favorable route was mapped out on the potential energy surface of the THP + Br reaction. The rate coefficients for all relevant steps involved in the reaction mechanism were computed using the energetics of coupled cluster calculations. On the basis of the results of the CCSD­(T)/CBS//B3LYP/cc-pVTZ level of theory, the calculated overall rate coefficients can be expressed as <i>k</i>_ov.,calc.(<i>T</i>) = 4.60 × 10^–10 exp­[−20.4 kJ mol^–1/(<i>RT</i>)] cm³ molecule^–1 s^–1 for the temperature range of 273–393 K. The calculated values are found to be in excellent agreement with the experimental data published previously