A quasiclassical trajectory study of product state distributions from the CN+H₂→HCN+H reaction

An extensive quasiclassical trajectory study of the dynamics of the CN+H₂→HCN+H reaction has been undertaken on two of the potential energy surfaces reported by ter Horst, Schatz, and Harding [J. Chem. Phys. 105, 558 (1996)] with the goal of converging product state distributions. The effect of zero-point energy violations on the behavior of the reactive cross section near threshold has been examined leading to an improved estimate of the thermal rate constant on ter Horst-Schatz-Harding potential energy surface 3 (3.01 ± 0.24 × 10^-14 cm³/s at 300 K). The calculated HCN vibrational product state distribution is not statistical and exhibits a systematic over population in the stretching vibrations of the ground state bend manifold indicating that the -C-N does not behave like a "spectator bond" in this reaction. There is also significant population in modes with bending excitation, but these vibrations are under populated relative to prior statistical expectations. The sensitivity of the distribution on the size of the barrier and its location in the entrance channel has been undertaken by comparing results on the ter Horst-Schatz-Harding potential energy surfaces 2 and 3. Similar to the case of exoergic atom-diatom reactions, it is found that the earlier barrier on ter Horst-Schatz-Harding potential energy surface 3 gives rise to more excitation in the -C-H stretching vibration. The rotational distributions of the HCN product appear similar to the thermal distribution of CN reagents from which they are born indicating that the abstraction of the light H atom perturbs the rotational motion of the cyano radical very little. The dependence of the average HCN rotational quantum number, 〈J〉, on the bending quantum number, υ₂, exhibits an interesting alternation such that the points for even values of υ₂ are larger than those for odd. There is a corresponding alternation in the dependence of the average scattering angle. 〈θ〉, on υ₂ in the opposite sense. These observations suggest that for the odd bending states (which are primarily l = 1) the energy diverted into exciting motion perpendicular to the reaction path at the transition state is not available to excite product rotation or to produce reactive trajectories with large impact parameters which lead to small scattering angles.