Effects of reactant rotational excitations on H2 + NH2 → H + NH3 reactivity

Abstract

Rotational mode specificity of the title reaction is examined using an initial state selected time-dependent wave packet method on an accurate ab initio based global potential energy surface. This penta-atomic reaction presents an ideal system to test several dynamical approximations, which might be useful for future quantum dynamics studies of polyatomic reactions, particularly with rotationally excited reactants. The first approximation involves a seven-dimensional (7D) model in which the two non-reactive N-H bonds are fixed at their equilibrium geometry. The second is the centrifugal sudden (CS) approximation within the 7D model. Finally, the J-shifting (JS) model is tested, again with the fixed N-H bonds. The spectator-bond approximation works very well in the energy range studied, while the centrifugal sudden and J-shifting integral cross sections (ICSs) agree satisfactorily with the coupled-channel counterparts in the low collision energy range, but deviate at the high energies. The calculated integral cross sections indicate that the rotational excitation of H2 somewhat inhibits the reaction while the rotational excitations of NH2 have little effect. These findings are compared with the predictions of the sudden vector projection model. Finally, a simple model is proposed to predict rotational mode specificity using K-averaged reaction probabilities.

The validities of centrifugal sudden approximations in chemical reaction dynamics

“…It is of great significance to carry out studies on the validities of the CS approximations in tetratomic and polyatomic systems, possibly combined with a full exploration of the PESs of reaction systems . To date, some efforts have been devoted to investigate the accuracy of the CS approximation in different tetraatomic and polyatomic systems . For example, the study of Lee and coworkers on the tetraatomic H 2 + D 2 reaction has first showed that the CS approximation can work well for the CID process but it cannot work well for the 4C and SE processes of this tetraatomic system.…”

“…The study on the H + H 2 O reaction showed that the CS approximation is inadequate for this tetraatomic reaction especially for its abstraction channel. In the time‐dependent wave packet study of polyatomic H 2 + NH 2 reaction, the 7D CS approximation agrees satisfactorily well with the CC calculation at low collision energy range but deviates at the high collision energies. The 6D and 7D quantum dynamics study on polyatomic OH + CH 4 reaction demonstrated that the CS approximation works reasonably in computing the integral cross sections over their considered energy range.…”

“…There has a growing interest in accurately computing the nonadiabatic dynamical quantities of benchmark chemical reactions on several potential energy surfaces (PESs) and in computing the corresponding quantities of tetraatomic and polyatomic chemical reactions. This research interest is motivated by the successful development of the nonadiabatic dynamical methods, in particular of quantum mechanical methods, , and by the great advances in quantum mechanical methods for tetraatomic and polyatomic reactions . However, the inclusion of multiple electronic states in the nonadiabatic treatment and the increase in atom numbers in a reactive system caused extremely difficulty to the quantum scattering calculations due to the rapid increase of basis functions, and a careful examination of CS approximations is essentially helpful in finding a balance between abating such computational difficulty and computational accuracy.…”

“…The adiabatic time‐dependent wave packet calculations were also carried out for the ion–molecule reactions of He + , D − + H 2 , H − + D 2 , as well as for the reaction of the neutral system of H + D 2 , with purpose to evaluate the validities of the CS approximation in these reaction dynamics. Meanwhile, there have relatively matured time‐dependent quantum wave packet methods suitable for the adiabatic study of tetraatomic and polyatomic reaction systems, which can be applied to probe the validities of the CS approximations for tetraatomic and polyatomic systems . Followed by the studies on validities of the CS approximations, especially those from Han's group, , there recently appeared a substantial number of dynamical studies comparing the CS and CC calculations, which are selectively presented in this review article.…”

Recent advances in quantum scattering calculations on polyatomic bimolecular reactions

“…In addition, all vibrational modes are less effective than translational energy in promoting the reaction. Due to the high computational cost for a full-dimensional quantum study for this reaction, a seven-dimensional (7D) quantum model with two non-reactive NH bonds fixed at their equilibrium geometry was also tested 181 . It was found that the spectator-bond approximation works very well in the energy range studied, while the CS and J-shifting ICSs agree satisfactorily with the coupled-channel results in the low collision energy region, but deviate at the high energies.…”