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

Abstract

Articles you may be interested inH+ versus D+ transfer from HOD+ to CO2: Bond-selective chemistry and the anomalous effect of bending excitation J. Chem. Phys. 134, 064312 (2011); 10.1063/1.3534908 Chemical reaction surface vibrational frequencies evaluated in curvilinear internal coordinates: Application to H + CH 4 H 2 + CH 3 Variational transition state theory and quasiclassical trajectory studies of the H 2 +OH→H+H 2 O reaction and some isotopic variantsAn extensive quasiclassical trajectory study of the dynamics of the CNϩH 2 →HCNϩH reaction has been undertaken on two of the potential energy surfaces reported by ter Horst, Schatz, and Harding ͓J. Chem. Phys. 105, 558 ͑1996͔͒ 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 ͑3.01Ϯ0.24ϫ10 Ϫ14 cm 3 /s at 300 K͒. 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, ͗J͘, on the bending quantum number, v 2 , exhibits an interesting alternation such that the points for even values of v 2 are larger than those for odd. There is a corresponding alternation in the dependence of the average scattering angle, ͗͘, on v 2 in the opposite sense. These observations suggest that for the odd bending states ͑which are primarily l ϭ 1͒ 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.

Reaction dynamics calculations for the CN+H2→HCN+H reaction: Applications of the rotating-bond approximation

“…8. As was seen in the QCT results, 18 the figure suggests that there is more correlation between the two stretching quantum numbers 1 and 3 than there is between 2 and either 1 and 3 ; this correlation is clear from the comparison between 2 ϭ0 and 2 ϭ1. ͓The correlation is less clear for 2 ϭ2, but here there is a relatively simple explanation.…”

“…In addition, we also compare the HCN vibrational distributions with QCT results 18 which have recently been reported by Bethardy et al for the same potential energy surface. The remainder of this article is organized as follows.…”

“…with different values of 2 indicates that this distribution can be approximated as a direct product of two probability functions, P͑ 1 , 2 , 3 ͒ϭP( 2 ) P( 1 , 3 ). 18 This also suggests that the reduced dimensionality approximation developed by Bowman,1,[11][12][13] where the CN rotational motion or HCN bending motion is treated adiabatically in the dynamics, would be a good approximation to predict the correlation of the two stretch vibrations.…”

“…We are also interested in the vibrational state distributions of HCN produced from the CNϩH 2 reaction where the reactants are governed by a Boltzmann distribution because QCT results of this type have recently been reported by Bethardy et al 18 The fraction of the corresponding products in vibrational state, f (T), is simply given by…”

Dynamics study of the OH + NH3 hydrogen abstraction reaction using QCT calculations based on an analytical potential energy surface

“…To note that this approach is similar to the ZP3 method of Nyman and Davidsson 20 and to the IEQMT approach of Varandas. 21 (c) Schatz et al 22,23 showed that in atom + triatom reactions this approach yields unphysically small cross-sections, and they suggested that the ZPE constraint should be applied just to the newly formed bond. In the present case, it is applied only to the H 2 O product.…”