The H + N2O → OH + N2 Reaction Dynamics on an Interpolated QCISD Potential Energy Surface. A Quasiclassical Trajectory Study

Abstract

A global ab initio interpolated potential-energy surface (PES) for the H + N2O → OH + N2 reaction has been constructed using the GROW package of Collins and co-workers. The ab initio calculations have been performed using the quadratic configuration interaction with single and double excitations (QCISD) and QCISD with quasipertubative treatment of triple excitations QCISD(T) methods. By use of this PES, a detailed quasiclassical trajectory study of integral and differential cross sections, product rovibrational populations and internal energy distributions are presented. It is found that the main mechanism for the title reaction is via an indirect process in which the H atom attaches to the N end of N2O and then undergoes a 1,3-hydrogen shift, leading to products. However, the direct abstraction process in which the H atom attacks the O end generating OH + N2 products accounts for ∼30% of the global reactivity at 1.5 eV collision energy. Both mechanisms contribute distinctly to the various reaction properties, and it is demonstrated that the trajectories associated with the direct mechanism have shorter collision times than those proceeding via the indirect mechanism. The theoretical integral cross sections as a function of collision energy are in good agreement with the experimental values at translational energies below 1.20 eV. At higher collision energies, the theoretical calculations on the present PES predict values of the cross sections smaller than those determined experimentally. The OH vibrational branching ratio OH(v‘ = 1)/OH(v‘ = 0) given by the theoretical calculations is considerably larger than that obtained experimentally by Brouard and co-workers. However, the calculated OH(v‘ = 0,1) rotational populations at 1.48 eV reproduce to a large extent the measurements. Also, a good accordance is found with the experimental OH state resolved differential cross sections at 1.48 eV. The calculated kinetic energy release distributions and triple scattering angle-recoil velocity differential cross sections for state-resolved OH(v‘,N‘) products show that a substantial fraction of the total energy goes into rotational excitation of the N2 coproduct due to the direct mechanism and that there exists a strong pair correlation in the angular and velocity distributions of the product molecules formed in coincidence.