Studies of the N(2D) + H2 reaction on revised potential energy surfaces

Energy-dependent stereodynamics for the H(2S)+NH(X3)→H2(X1Σg+)+N(4S) reaction on the improved ZH potential energy surface

In this work, quasi-classical trajectory (QCT) calculations have been first carried out for the title reaction on a new global potential energy surface for the lowest quartet electronic state, 4A″. The average rotational alignment factor [P2(j'·k)] as a function of collision energy and the two commonly used polarization dependent generalized differential cross sections PDDCS00, PDDCS20, have been calculated in the center-of-mass (CM) frame, separately. Three angular distributions, P(θr), P(φr), and P(θr, φr) are also calculated to gain insight into the alignment and the orientation of the product molecules. Calculations show that the average rotational alignment factor on the ZH PES is almost invariant with collision energies. The distributions of P(θr) and P(φr) derived from the title reaction indicate that the product polarization is strong. Validity of the QCT calculation has been examined and proven in the comparison with the quantum-wave-packet calculation results. Comparisons with available quasi-classical trajectory results are made and discussed.

QUASICLASSICAL TRAJECTORY STUDY OF PRODUCT POLARIZATIONS IN THE H(2S) + NH → N(4S) + H2 REACTION

Based on the DMBE potential surface of the 4 A ″ ground-state, the product rotational polarizations in the title reaction are studied by using quasiclassical trajectory (QCT) calculation method. Three angular distributions of P(θr), P(Φr), P(θr, Φr) and the four polarization-dependent differential cross sections (PDDCSs) were calculated for the collision energy range of 1–20 kcal/mol. The results revealed that the product is backward-scattering and the product rotational angular momentum j′ is aligned and oriented. With the increment of collision energy, the degree of the product alignment and orientation is enhanced, showing the collision energy-dependent behaviors of the product polarizations.

Differential and Integral Cross Sections of the N(2D) + H2 → NH + H Reaction from Exact Quantum and Quasi-Classical Trajectory Calculations

Exact quantum mechanical state-to-state differential and integral cross sections and their energy dependence have been calculated on an accurate NH2 potential energy surface (PES), using a newly proposed Chebyshev wave packet method. The NH product is found to have a monotonically decaying vibrational distribution and an inverted rotational distribution. The product angular distributions peak in both forward and backward directions, but with a backward bias. This backward bias is more pronounced than that observed previously on a less accurate PES. Both the differential and integral cross sections oscillate mildly with collision energy, indicating the dominance of short-lived resonances. The quantum mechanical results are compared with those obtained from quasi-classical trajectories. The agreement is generally reasonable and the discrepancies can be attributed to the neglect of quantum effects such as tunneling. Detailed analysis of the trajectories revealed that the backward bias in the differential cross section stems overwhelmingly from the fast insertion component of the reaction, augmented with some flux from the abstraction channel, particularly at high collision energies.

Influence of rotation and isotope effects on the dynamics of the N(D2)+H2 reactive system and of its deuterated variants

Integral cross sections and thermal rate constants have been calculated for the N((2)D)+H(2) reaction and its isotopic variants N((2)D)+D(2) and the two-channel N((2)D)+HD by means of quasiclassical trajectory and statistical quantum-mechanical model methods on the latest ab initio potential-energy surface [T.-S. Ho et al., J. Chem. Phys. 119, 3063 (2003)]. The effect of rotational excitation of the diatom on the dynamics of these reactions has been investigated and interesting discrepancies between the classical and statistical model calculations have been found. Whereas a net effect of reagent rotation on reactivity is always observed in the classical calculations, only a very slight effect is observed in the case of the asymmetric N((2)D)+HD reaction for the statistical quantum-mechanical method. The thermal rate constants calculated on this Potential-Energy Surface using quasiclassical trajectory and statistical model methods are in good agreement with the experimental determinations, although the latter are somewhat larger. A reevaluation of the collinear barrier of the potential surface used in the present study seems timely. Further theoretical and experimental studies are needed for a full understanding of the dynamics of the title reaction.

A Quantum Wave Packet Dynamics Study of the N(2D) + H2 Reaction

We report a dynamics study of the reaction N((2)D) + H(2) (v=0, j=0-5) --> NH + H using the time-dependent quantum wave packet method and a recently reported single-sheeted double many-body expansion potential energy surface for NH(2)(1(2)A' ') which has been modeled from accurate ab initio multireference configuration-interaction calculations. The calculated probabilities for (v=0, j=0-5) are shown to display resonance structures, a feature also visible to some extent in the calculated total cross sections for (v=0, j=0). A comparison between the calculated centrifugal-sudden and coupled-channel reaction probabilities validate the former approximation for the title system. Rate constants calculated using a uniform J-shifting scheme and averaged over a Boltzmann distribution of rotational states are shown to be in good agreement with the available experimental values. Comparisons with other theoretical results are also made.

Computational study of the reaction of N(D2) atoms with CH2F radicals: An example of a barrier-free reaction involving very high internal energies

The singlet potential-energy surface for the N(2D)+CH2F(2A') reaction has been studied employing both second-order Møller-Plesset and density-functional theories. The energies of the involved species have been refined using the Gaussian-2, complete basis set, and coupled-cluster singles and doubles (triples) methods. The reaction proceeds through the formation of an initial intermediate, which does not involve any activation barrier. Based on the energy profile for the singlet potential-energy surface, the preferred product should be the most exothermic one, namely, HCN+HF, followed by HNC+HF and FCN+H2. This result seems in contradiction with a computational study of the kinetics of the title reaction in terms of the statistical theories, which leads to the prediction that the production of HNC+HF should be the dominant channel. Consequently, a limited molecular-dynamics study has been carried out, concluding that in fact the system behaves in a nonstatistical way. According to the molecular-dynamics study, the most exothermic channel, HCN+HF, should be the dominant one. An analysis of the possible role of the singlet surface in the reaction of N(4S) with CH2F(2A') has also been carried out. The computational study shows that the microcanonical coefficients for the nonadiabatic channels are much smaller than the competing adiabatic ones. Therefore, the reaction of N(4S) with CH2F(2A') should proceed on the triplet surface without spin change.

Repulsive double many-body expansion potential energy surface for the reactions N(4S)+ H2⇌ NH(X3Σ–)+ H from accurate ab initio calculations

A single-sheeted DMBE potential energy surface is reported for the reactions N(4S)+H2<-->NH(X3Sigma-)+H based on a fit to accurate multireference configuration interaction energies. These have been calculated using the aug-cc-pVQZ basis set of Dunning and the full valence complete active space wave function as reference, being semi-empirically corrected by scaling the two-body and three-body dynamical correlation energies. The topographical features of the novel global potential energy surface are examined in detail, including a conical intersection involving the two first 4A'' potential energy surfaces which has been transformed into an avoided crossing in the present single-sheeted representation.