Ab initio molecular orbital and density functional characterization of the potential energy surface of the N2O+Br reaction

Abstract

The lowest doublet (2A′) potential energy surface for the reaction N2O+Br→N2+OBr was investigated using ab initio and nonlocal density functional theory calculations. Geometries, energies and vibrational frequencies for stationary points were evaluated at HF/6-31G(d), MP2/6-31G(d) and B3LYP/6-31G(d) levels of theory. All levels of calculation give a similar geometry for the transition state, but the MP2 barrier is narrower. Intrinsic reaction coordinate (IRC) calculations starting from the transition state and proceeding toward the two local minima confirmed that IRC trajectories reached the reactant and product regions, respectively. Calculations of kinetic isotope effects were also performed. They are influenced by the theoretical level and the B3LYP method gives results in best agreement with experimental data. The HF method predicts the relative values of both the primary and secondary nitrogen kinetic isotope effects less accurately. At the MP2 level of calculations only the oxygen kinetic isotope effect is reproduced satisfactorily. Finally, the best value for the activation energy is again provided by the B3LYP method.