Potential energy surface for the CH3+HBr→CH4+Br hydrogen abstraction reaction: Thermal and state-selected rate constants, and kinetic isotope effects

Abstract

The gas-phase hydrogen abstraction title reaction was carefully investigated. First, ab initio molecular orbital theory was used to study the stationary points along the reaction path: reactants, hydrogen-bonded complex, saddle point, and products. Optimized geometries and harmonic vibrational frequencies were calculated at the second-order Møller–Plesset perturbation theory level, and then single-point calculations were performed at a higher level of calculation: coupled-cluster with triple-zeta basis set. The effects of the level of calculation, zero-point energy (ZPE), thermal corrections [TC (298.15 K)], spin–orbit coupling, and basis set superposition error (BSSE) on the energy changes were analyzed. It was concluded that at room temperature (i.e., with ZPE and TC), when the BSSE was included, the complex disappears and the activation enthalpy is +0.39 kcal mol−1 above the reactants. Second, an analytical potential energy surface was constructed with suitable functional forms to represent vibrational modes, and was calibrated by using experimental and theoretical stationary point properties and the tendency of the kinetic isotope effects. On this surface, the forward and reverse thermal rate constants were calculated using variational transition state theory with semiclassical transmission coefficients over a wide temperature range. In both cases, we found a direct dependence on temperature and, therefore, positive activation energies. The influence of the tunneling factor was very small due to the flattening of the surface in the entrance valley. This surface was also used to analyze dynamical features, such as reaction-path curvature, the coupling between the reaction coordinate and vibrational modes, and the effect of vibrational excitation on the rate constants. It was found that excitation of the BrH stretching mode enhances the forward reaction, whereas the excitation of the CH3 umbrella mode has the opposite effect.