Potential energy surface and quasiclassical trajectory studies of the CN+H-2 reaction

We present dynamical studies of the CN+H-2 reaction based on an empirical potential energy surface that is derived from high quality ab initio calculations. The ab initio calculations, which use a multireference configuration interaction method with large correlation consistent basis sets, indicate that the linear HHCN barrier is about 4.3 kcal/mol above CN+H-2, and that there is no reaction path which connects CN+H-2 to the stable intermediate H2CN, although there is a path for dissociation of H2CN to H+HCN. The empirical surface is written as a sum of two-, three-, and four-body terms, with the two- and three-body terms for HCN based on an accurate global surface that describes both the HCN and HNC force fields. The four-body terms are developed so as to describe the HHCN linear saddle point and the H2CN minimum accurately, as well as dissociation of H2CN into HCN+H, and the ridge which separates the abstraction and H2CN dissociation pathways. Other features of the potential surface, such as the HCNH cis and trans minima, and the pathways leading to the formation of HNC+H are also described, though less accurately. Three different choices for the HHCN saddle point properties are considered. We find that the surface which matches the ab initio barrier energy most accurately gives rate constants that are too low. Much better agreement is obtained using a 3.2 kcal/mol barrier. The trajectory results show typical dependence of the CN+H-2 reactive cross sections on initial translational energy and initial vibration/rotation state, with CN behaving as a spectator and H-2 playing an active role in the reaction dynamics. Analysis of the H+HCN products indicates that both the C-H stretch and bend modes are significantly excited, with bend excitation showing strong sensitivity to the saddle point properties and to reagent translational energy. At translational energies below 20 kcal/mol, direct H abstraction is strongly favored over addition elimination. (C) 1996 American Institute of Physics.