An Interpolated ab Initio Quantum Scattering Study of the Temperature Dependence of the CH3 + HBr → CH4 + Br Reaction

Abstract

An interpolated ab initio quantum scattering (AIQS) study for the CH3 + HBr → CH4 + Br reaction is presented. The ab initio calculation of energy points has been done using the UMP2/6-311G(3df,d,p) level of theory. Interpolation between the energy points was done using a recently developed generalized discrete variable representation (GDVR) approach. The quantum scattering calculations were performed using a reduced dimensionality rotating line umbrella (RLU) model. The UMP2 results give a vibrationally adiabatic ground-state barrier height of 0.69 kcal/mol for the forward reaction. An unstable van der Waals complex CH3···HBr was also obtained. This complex plays only a minor role in the reaction dynamics. Calculated thermal rate coefficients show a strong nonlinear Arrhenius behavior. Below 550 K they have a negative temperature dependence, whereas a positive activation energy was obtained at higher temperatures. It was found that placing one quantum of energy in the vibrational mode of the H−Br bond str...